* fix(prometheus): skip custom metadata labels a metric already carries
A custom metadata label whose sanitized name matches one of a metric's
built-in labels was appended a second time, so every .labels() call on
that metric raised ValueError. With custom_prometheus_metadata_labels:
[project_alias], the per-project per-model rate limit gauges raised on
every request from a project key with per-model limits, and the rest of
the success event (latency, deployment success, total requests) was
skipped.
* test(prometheus): cover shared custom labels on every endpoint, failure, metadata and outage burst
* test(prometheus): drain the shared upstream at the start of each integration test
* test(prometheus): assert the forwarded route, model and stream per endpoint cell
---------
Co-authored-by: yucheng-berri <293926326+yucheng-berri@users.noreply.github.com>
