[sdk-metrics] Exemplar spec improvements

[CodeBlanch]

[Split out from #5364]

Changes

ExemplarReservoir

• Offer method signature no longer includes the "index" parameter which is not part of the spec. Instead it now accepts struct ExemplarMeasurement which only exposes the value & tags (to match spec) but also tracks explicit histogram index as an internal thing.
• Collect method signature no longer include "actualTags" or "reset" which are not part of the spec. The "reset" flag has become bool ResetOnCollect property on ExemplarReservoir. Tag filtering is now handled by struct ReadOnlyFilteredTagCollection which is exposed on Exemplar. Return type changed from Exemplar[] to struct ReadOnlyExemplarCollection.

Exemplar

• Return type of FilteredTags property is now struct ReadOnlyFilteredTagCollection instead of List<KeyValuePair<string, object?>>?.
• Now exposes long LongValue in addition to double DoubleValue.

MetricPoint

• Exemplar[] GetExemplars() is now bool TryGetExemplars(out ReadOnlyExemplarCollection? exemplars).

Misc improvements

• ReadOnlyExemplarCollection when enumerated only returns Exemplars which have been updated (Timestamp != default). Previously each exporter had to understand the behavior and check that.
• Removed nasty Linq & allocations from Exemplar.FilteredTags.
• Introduced internal FixedSizeExemplarReservoir to house the shared logic both SimpleFixedSizeExemplarReservoir & AlignedHistogramBucketExemplarReservoir share.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 66.22222% with 76 lines in your changes are missing coverage. Please review.

Project coverage is 83.25%. Comparing base (6250307) to head (b10a2c9).
Report is 99 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5386      +/-   ##
==========================================
- Coverage   83.38%   83.25%   -0.14%     
==========================================
  Files         297      284      -13     
  Lines       12531    12049     -482     
==========================================
- Hits        10449    10031     -418     
+ Misses       2082     2018      -64     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	83.17% <66.22%> (?)
unittests-Solution-Stable	83.10% <66.22%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/OpenTelemetry/Metrics/AggregatorStore.cs	82.08% <100.00%> (+1.69%)	⬆️
...xemplar/AlignedHistogramBucketExemplarReservoir.cs	80.00% <100.00%> (-12.86%)	⬇️
...nTelemetry/Metrics/Exemplar/ExemplarMeasurement.cs	100.00% <100.00%> (ø)
...penTelemetry/Metrics/Exemplar/ExemplarReservoir.cs	100.00% <100.00%> (ø)
...Telemetry/Metrics/MetricPointOptionalComponents.cs	100.00% <100.00%> (+22.22%)	⬆️
src/OpenTelemetry/ReadOnlyFilteredTagCollection.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/ReadOnlyTagCollection.cs	100.00% <100.00%> (ø)
...try/Metrics/Exemplar/FixedSizeExemplarReservoir.cs	96.77% <96.77%> (ø)
...trics/Exemplar/SimpleFixedSizeExemplarReservoir.cs	87.50% <90.00%> (ø)
src/OpenTelemetry/Metrics/Exemplar/Exemplar.cs	91.11% <91.11%> (-8.89%)	⬇️
... and 4 more

... and 48 files with indirect coverage changes

[cijothomas]

Thanks a lot!
left couple of minor comments, lets address it as follow ups