fix(semconv): attach spec-mandated explicit bucket boundaries to GenAI histogram helpers

[alliasgher]

Description

The four GenAI histogram helpers in opentelemetry-semantic-conventions called meter.create_histogram without passing explicit_bucket_boundaries_advisory. The SDK therefore fell back to _DEFAULT_EXPLICIT_BUCKET_HISTOGRAM_AGGREGATION_BOUNDARIES, which is tuned for request-duration metrics in the seconds range and produces unusable histograms for latency-per-token and TTFT metrics — the exact problem flagged in the semconv spec, which says these metrics SHOULD be specified with ExplicitBucketBoundaries.

Pass the semconv-prescribed boundaries for all four helpers:

• gen_ai.client.operation.duration, gen_ai.server.request.duration, gen_ai.server.time_to_first_token share the latency boundary set [0.01 … 81.92] seconds.
• gen_ai.server.time_per_output_token uses the per-token boundary set [0.01 … 2.5] seconds.

Fixes #4946

Checklist

• pytest opentelemetry-semantic-conventions/tests/test_gen_ai_metrics.py
• New test file asserts each factory passes the correct explicit_bucket_boundaries_advisory to Meter.create_histogram
• CHANGELOG entry added

[alliasgher]

Fixed the two CI failures:

check-links: The CHANGELOG had a stale link pointing to opentelemetry-python-contrib/pull/5076 (404) — the correct URL is opentelemetry-python/pull/5076 (matching main). Fixed in the latest commit.

pypy-3.10 Windows: The failing test TestConcurrentMultiSpanProcessor::test_force_flush_late_by_timeout is in opentelemetry-sdk/tests/trace/test_span_processor.py, which is unrelated to this PR's changes (this PR only touches opentelemetry-semantic-conventions/src/opentelemetry/semconv/metrics/gen_ai_metrics.py and its test file). The failure appears to be a pre-existing race-condition flake specific to the pypy-3.10 Windows environment.

[tammy-baylis-swi]

Overall lgtm, and this dir isn't generally tested.

It would be good if @open-telemetry/python-approvers who make regular GenAI instrumentation changes could also look at this please.

[MikeGoldsmith]

Looks good, thanks @alliasgher - I've left a suggestion to update the changelog to use the PR number instead of issue number.

[MikeGoldsmith]

Suggested change

	- `opentelemetry-semantic-conventions`: Attach spec-mandated explicit bucket boundaries to the GenAI histogram helpers (`gen_ai.client.operation.duration`, `gen_ai.server.request.duration`, `gen_ai.server.time_to_first_token`, `gen_ai.server.time_per_output_token`); without them the default SDK buckets produced unusable histograms for latency-per-token metrics
	([#4946](https://github.com/open-telemetry/opentelemetry-python/issues/4946))
	- `opentelemetry-semantic-conventions`: Attach spec-mandated explicit bucket boundaries to the GenAI histogram helpers
	([#5084](https://github.com/open-telemetry/opentelemetry-python/issues/5084))

[alliasgher]

Done.

[lmolkova]

I support the cause, but this code is auto-generated and changes will disappear with the next update. this is currently blocked on open-telemetry/semantic-conventions#1225.

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