Fix: filter exemplar for observable instrument and export of exemplar without trace and span ids

[fcollonval]

Description

Fixes #4250

• It applies correctly the exemplar filter to the observable instruments
• It does not export the trace_id and span_id attributes of exemplars if undefined
• It fixes the type of trace_id and span_id in exemplar classes

The two latter fixes were extracted from #4178

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Check example docs/examples/metrics/instruments/example.py
• Add tests by @xrmx in exporter/opentelemetry-exporter-otlp-proto-common/tests/test_metrics_encoder.py
• Extend test to check consume_measurement is called with exemplar filter result for async instrument

Does This PR Require a Contrib Repo Change?

• [ ] Yes. - Link to PR:
• [x] No.

Checklist:

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added
• ~Documentation has been updated~

[fcollonval]

Thanks @xrmx for the quick review - I don't know what to do about the remaining failure for the generate-workflows job.

[emdneto]

Thanks @xrmx for the quick review - I don't know what to do about the remaining failure for the generate-workflows job.

~We need to update the workflows. I can fix it for you~. It seems we have another issue with internal generate-workflows-lib. Fix here https://github.com/open-telemetry/opentelemetry-python-contrib/pull/2964

[xrmx]

@fcollonval I think we are also missing tests for the OTEL_METRICS_EXEMPLAR_FILTER env var, when I set OTEL_METRICS_EXEMPLAR_FILTER=always_off, the correct sampler is set but I get exemplars in the exported metrics but this is fixed by this PR too.

UPDATE: maybe the test requested by Aaron is enough

[xrmx]

I've started writing some integration tests but the always_off and trace_based are wrong because there should be an exemplar in the trace_based one: https://github.com/open-telemetry/opentelemetry-python/commit/056313cfe738d0aa8d3a0f7e2ee07102d1288d09

[xrmx]

@aabmass Emidio fixed tests PTAL

[fcollonval]

Thanks a lot to all of you for helping fixing this and more generally for the maintenance of this great repo.