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

Signed-off-by: Ali alliasgher123@gmail.com

[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.