POC: More simply set attribute on exemplar spans

[samwright]

Proof of Concept for a way to support metric exemplars and tail-based sampling at the same time. Hopefully this will spur discussion on how we want to support this use-case.

This is a simpler version of #5915

Relevant issue: open-telemetry/opentelemetry-specification#2922

This makes a few big changes:

• HistogramExemplarReservoir and RandomFixedSizeExemplarReservoir now collect the first instead of the last exemplar sample they see. This lets us know whether or not a span will be used as an exemplar while it is still current.
• Suggest otel.exemplar as a potential new attribute that, when set to true means the sample is being used as an exemplar.

This approach allows tail-based samplers to ensure that all traces containing spans with otel.exemplar == true are kept, while the rest can be sampled without accidentally dropping a span used as an exemplar.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
.../internal/exemplar/FixedSizeExemplarReservoir.java	89.28% <100.00%> (ø)
.../internal/exemplar/HistogramExemplarReservoir.java	90.90% <ø> (ø)
...nal/exemplar/RandomFixedSizeExemplarReservoir.java	83.33% <100.00%> (ø)
...y/sdk/metrics/internal/exemplar/ReservoirCell.java	90.00% <92.30%> (-0.48%)	⬇️

... and 3 files with indirect coverage changes

📢 Thoughts on this report? Let us know!.

[jack-berg]

Closing for same reason as this comment. Feel free to re-open if I've misunderstood something!