fix: Counter adds the delta from last collection.

[TommyCpp]

Fix #394

During each collection interval. The processor will maintain two places to store the values. cumulative and current. current won't get clear until the next time instrument gets updated. So if the instrument hasn't updated since the last collection. We should skip any change made to the cumulative part.

[codecov]

Codecov Report

Merging #395 (a92acd0) into master (1bcbe77) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
- Coverage   49.31%   49.30%   -0.01%     
==========================================
  Files          66       66              
  Lines        5376     5377       +1     
==========================================
  Hits         2651     2651              
- Misses       2725     2726       +1     

Impacted Files	Coverage Δ
opentelemetry/src/sdk/metrics/processors/basic.rs	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1bcbe77...a92acd0. Read the comment docs.

[jtescher]

Good catch