Add prometheus exporter benchmark

[dmathieu]

This adds benchmarks for the prometheus exporter, with different numbers of metrics.

[dmathieu]

Benchmarks results on a 2019 Macbook Pro:

open-telemetry/opentelemetry-go/exporters/prometheus› git:(prometheus-benchmark) go test -bench=.
2022/09/21 14:17:44 "invalid.gauge.name" is not a valid metric name
2022/09/21 14:17:44 "invalid.counter.name" is not a valid metric name
2022/09/21 14:17:44 "invalid.hist.name" is not a valid metric name
goos: darwin
goarch: amd64
pkg: go.opentelemetry.io/otel/exporters/prometheus
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkCollect1-12 76323 16575 ns/op
BenchmarkCollect10-12 37503 31560 ns/op
BenchmarkCollect100-12 6524 182864 ns/op
BenchmarkCollect1000-12 649 1752732 ns/op
BenchmarkCollect10000-12 55 18618929 ns/op
PASS
ok go.opentelemetry.io/otel/exporters/prometheus 6.992s

[codecov]

Codecov Report

Merging #3213 (db70ad7) into main (bfaff3a) will decrease coverage by 0.0%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3213     +/-   ##
=======================================
- Coverage   77.3%   77.3%   -0.1%     
=======================================
  Files        159     159             
  Lines      11139   11139             
=======================================
- Hits        8617    8615      -2     
- Misses      2325    2327      +2     
  Partials     197     197             

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	81.1% <0.0%> (-0.9%)	⬇️