Add the sdk/metric/internal/exemplar package

[MrAlias]

Part of #559

Add the implementation package of of OpenTelemetry metric exemplars reservoirs skeleton.

This PR is split from #4455 to reduce the size of changes being evaluated. It is meant to be followed up with changes included in that PR:

• add the drop reservoir
• add the fixed size reservoir
• add the histogram reservoir
• add the trace-based sampler reservoir/filter
• integrate this new package into the metric pipeline as an experimental feature:

Lack of ExemplarFilter

The ExemplarFilter interface currently defined in the specification is not included in this package. This is because open-telemetry/opentelemetry-specification#3820 removes this type and is expected to be merged.

If open-telemetry/opentelemetry-specification#3820 fails to merge, or is modified to not remove the ExemplarFilter, it can be added back into this package (i.e. revert c4d8381).

Offer dropped attributes

The Offer method of the ExemplarReservoir is defined to accept the full set of attributes a measurement is made with. However, there is an optional provision below this definition stating that it can also accept just the dropped attributes. This package defines the method to accept just the dropped attributes.

The reason for this decision is based on the fact that attribute.Set's Filter method returns these values at no additional cost when the set is filtered. See this comment describing the performance improvement numbers.

[MrAlias]

cc @jsuereth

[codecov]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (cef39a1) 82.3% compared to head (b4f8613) 82.3%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4835   +/-   ##
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  Coverage   82.3%   82.3%           
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  Files        226     226           
  Lines      18481   18557   +76     
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+ Hits       15222   15286   +64     
- Misses      2973    2983   +10     
- Partials     286     288    +2     

Files	Coverage Δ
sdk/metric/cache.go	100.0% <100.0%> (ø)
sdk/metric/meter.go	90.4% <81.6%> (-2.2%)	⬇️

... and 1 file with indirect coverage changes