Avoid exemplar allocations if there are no measurements

[jack-berg]

This change is part of the #5178 but can be considered separately.

I noticed that a decent amount of allocations happen in exemplar classes even when exemplars are disabled. The culprit is that we when collecting, we allocate a new ArrayList<>() even when a reservoir has received no measurements.

Performance before:

Benchmark                                                                            (aggregationGenerator)  (aggregationTemporality)  Mode  Cnt           Score           Error   Units
HistogramCollectBenchmark.recordAndCollect                                        EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5  3390255558.400 ±  29491981.021   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate                         EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5           3.007 ±         0.020  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm                    EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5    10689731.200 ±     36579.743    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                              EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time                               EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5           2.000                      ms
HistogramCollectBenchmark.recordAndCollect                                        EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5  3436534566.600 ± 102736943.608   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate                         EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           3.202 ±         0.199  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm                    EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5    11536620.800 ±    526953.810    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                              EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time                               EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           3.000                      ms
HistogramCollectBenchmark.recordAndCollect                             DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5  9538995141.800 ±  95720490.562   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate              DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5           0.636 ±         0.030  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm         DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5     6363548.800 ±    305261.174    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                   DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5             ≈ 0                  counts
HistogramCollectBenchmark.recordAndCollect                             DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5  9042829725.000 ± 169838553.573   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate              DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           0.933 ±         0.068  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm         DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5     8849014.400 ±    654075.369    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                   DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time                    DEFAULT_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                      ms
HistogramCollectBenchmark.recordAndCollect                      ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5  2616061300.000 ±  67260283.355   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate       ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5           2.391 ±         0.104  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm  ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5     6559592.000 ±    284720.892    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count            ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5             ≈ 0                  counts
HistogramCollectBenchmark.recordAndCollect                      ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5  2624043408.200 ±  48730935.089   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate       ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           3.420 ±         0.167  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm  ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5     9411476.800 ±    308800.163    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count            ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time             ZERO_MAX_SCALE_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                      ms

And after:

Benchmark                                                                                  (aggregationGenerator)  (aggregationTemporality)  Mode  Cnt           Score           Error   Units
HistogramCollectBenchmark.recordAndCollect                                              EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5  3375821208.400 ±  30739355.578   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate                               EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5           2.799 ±         0.022  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm                          EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5     9908505.600 ±     38683.221    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                                    EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time                                     EXPLICIT_BUCKET_HISTOGRAM                     DELTA    ss    5           2.000                      ms
HistogramCollectBenchmark.recordAndCollect                                              EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5  3414353175.200 ±  24518270.279   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate                               EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           3.061 ±         0.157  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm                          EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5    10958736.000 ±    509633.147    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                                    EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time                                     EXPLICIT_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           2.000                      ms
HistogramCollectBenchmark.recordAndCollect                             DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5  9523546925.000 ± 109660916.879   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate              DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5           0.605 ±         0.032  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm         DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5     6041968.000 ±    367044.332    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                   DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5             ≈ 0                  counts
HistogramCollectBenchmark.recordAndCollect                             DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5  9120267599.800 ±  81938280.584   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate              DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           0.904 ±         0.069  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm         DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5     8648201.600 ±    709467.779    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count                   DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time                    DEFAULT_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           2.000                      ms
HistogramCollectBenchmark.recordAndCollect                      ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5  2703226991.600 ± 143524730.218   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate       ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5           2.107 ±         0.140  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm  ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5     5973420.800 ±    300748.546    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count            ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                     DELTA    ss    5             ≈ 0                  counts
HistogramCollectBenchmark.recordAndCollect                      ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5  2569519275.400 ± 130020912.482   ns/op
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate       ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           3.119 ±         0.125  MB/sec
HistogramCollectBenchmark.recordAndCollect:·gc.alloc.rate.norm  ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5     8403596.800 ±    301287.233    B/op
HistogramCollectBenchmark.recordAndCollect:·gc.count            ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                  counts
HistogramCollectBenchmark.recordAndCollect:·gc.time             ZERO_MAX_SCALE_BASE2_EXPONENTIAL_BUCKET_HISTOGRAM                CUMULATIVE    ss    5           1.000                      ms

A nice improvement, probably on the order of ~10%.

[codecov]

Codecov Report

Base: 91.07% // Head: 91.04% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (b0818b6) compared to base (feb0297).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5182      +/-   ##
============================================
- Coverage     91.07%   91.04%   -0.04%     
+ Complexity     4868     4867       -1     
============================================
  Files           548      548              
  Lines         14373    14379       +6     
  Branches       1378     1378              
============================================
+ Hits          13090    13091       +1     
- Misses          891      893       +2     
- Partials        392      395       +3     

Impacted Files	Coverage Δ
.../internal/exemplar/FixedSizeExemplarReservoir.java	89.28% <100.00%> (-1.63%)	⬇️
...y/sdk/metrics/internal/exemplar/ReservoirCell.java	90.47% <0.00%> (-9.53%)	⬇️
...telemetry/sdk/trace/export/BatchSpanProcessor.java	94.48% <0.00%> (-0.69%)	⬇️
...nternal/state/DefaultSynchronousMetricStorage.java	93.10% <0.00%> (+1.72%)	⬆️

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