PoC: HistogramReservoir uses a time-weighted algorithm

[dashpole]

PoC of open-telemetry/opentelemetry-specification#4678.

Please only pay attention to 9429757. Previous commits are from #7447 and #7443, and are important for having real performance numbers.

Compared with #7443, this does come with a performance cost according to our benchmarks to track when to record the next measurement:

                           │ fixedsize.txt │               new.txt               │
                           │    sec/op     │    sec/op     vs base               │
HistogramReservoirOffer-24     23.58n ± 4%   36.81n ± 10%  +56.11% (p=0.002 n=6)

The percentage cost increase is large (~50%), but the absolute cost is relatively small (~10ns). This increase is largely because of how the benchmark is written: it benchmarks the first 100 measurements by resetting the count every 100 observations.

If we do not reset storage every 100 observations in the benchmark, the rate of recording exemplars becomes essentially zero. So for histograms with very large numbers of observations, this is actually a substantial performance improvement:

                           │ fixedsize.txt │               new.txt               │
                           │    sec/op     │    sec/op     vs base               │
HistogramReservoirOffer-24    23.580n ± 4%   5.189n ± 14%  -78.00% (p=0.002 n=6)

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.7%. Comparing base (a10652b) to head (34911fb).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/metric/exemplar/fixed_size_reservoir.go	88.6%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7458   +/-   ##
=====================================
  Coverage   85.7%   85.7%           
=====================================
  Files        284     284           
  Lines      25017   25054   +37     
=====================================
+ Hits       21441   21473   +32     
- Misses      3197    3200    +3     
- Partials     379     381    +2     

Files with missing lines	Coverage Δ
sdk/metric/exemplar/histogram_reservoir.go	94.8% <100.0%> (+2.5%)	⬆️
sdk/metric/exemplar/storage.go	96.4% <100.0%> (-3.6%)	⬇️
sdk/metric/exemplar/fixed_size_reservoir.go	95.7% <88.6%> (-2.0%)	⬇️

... and 2 files with indirect coverage changes

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[dashpole]

Closing, as it is just a PoC