[Bugfix][Router] Update average decoding duration stats

[dsxyy]

Summary

• update decoding_length_monitors when a request completes
• measure decoding duration as completion time minus first-token time, matching the existing RequestStats.avg_decoding_length definition
• expire decoding-duration samples outside the configured sliding window
• add a regression test covering pre-completion, completed, and expired samples

Problem

decoding_length_monitors was initialized and read by get_request_stats(), but never updated, so avg_decoding_length remained -1 after successfully completed requests. The existing source definition describes this value as the time from first token to completion, and first_token_time was already recorded but otherwise unused.

Testing

• pytest -q src/tests/test_request_stats.py src/tests/test_singleton.py src/tests/test_stale_metrics.py src/tests/test_metrics.py src/tests/test_instance_failover.py src/tests/test_request_validation.py src/tests/test_transcription_streaming.py (32 passed)
• Router regression selection (68 passed)
• black --check
• isort --check-only
• ruff check src/tests/test_request_stats.py
• codespell
• git diff --check

[gemini-code-assist]

Code Review

This pull request implements tracking for average decoding length statistics in RequestStatsMonitor and adds corresponding unit tests. The reviewer suggested using pop instead of get when retrieving first_token_time upon request completion to prevent potential memory leaks.