RSPEED-3017: use custom buckets for response duration histogram

[major]

Description

Add custom bucket boundaries to the response_duration_seconds Prometheus histogram. The default prometheus_client buckets max out at 10s, which caps Grafana histogram_quantile calculations for requests that take longer. This reuses the existing LLM_INFERENCE_DURATION_BUCKETS tuple (0.1-120s) already used by llm_inference_duration_seconds.

Type of change

• Refactor
• New feature
• Bug fix
• CVE fix
• Optimization
• Documentation Update
• Configuration Update
• Bump-up service version
• Bump-up dependent library
• Bump-up library or tool used for development (does not change the final image)
• CI configuration change
• Konflux configuration change
• Unit tests improvement
• Integration tests improvement
• End to end tests improvement
• Benchmarks improvement

Tools used to create PR

• Assisted-by: Claude (opencode)
• Generated by: N/A

Related Tickets & Documents

• RSPEED-3017

Checklist before requesting a review

• I have performed a self-review of my code.
• PR has passed all pre-merge test jobs.
• If it is a core feature, I have added thorough tests.

Testing

• All 16 existing metrics unit tests pass (uv run make test-unit).
• Verified response_duration_seconds histogram now uses the custom buckets (0.1, 0.5, 1.0, 2.5, 5.0, 10.0, 20.0, 30.0, 60.0, 120.0, +Inf) matching the llm_inference_duration_seconds histogram.
• uv run make verify passes cleanly.

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📥 Commits

Reviewing files that changed from the base of the PR and between d15d37d and 047091b.

📒 Files selected for processing (1)

• src/metrics/__init__.py

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[tisnik]

LGTM