Xinference user guide metrics page

Operator-facing metrics documentation, corroborated against xinference/core/metrics.py on 2026-10-05. doc/templates/metrics.jinja is the template it is generated from, so a doc/code mismatch is possible in principle; the metric names were confirmed against the code, not the doc. | 2026-10-05 PATH SWEEP: STALE PATH FIXED. 'core/metrics.py' is ambiguous inside this repo and resolves to nothing at the root; the real file is xinference/core/metrics.py, confirmed via the authenticated contents API (49,032 bytes). The sibling xinference/router/metrics.py is the other metrics module on that repo and is NOT the one this record is about - noted so a future sweep does not swap them.