feat(alerts): the metric catalog is written by what emits it (#5061)

#5061 "Metric catalog belongs to what emits: plugins register their
metrics; names match what is recorded".

- The static METRIC_CATALOG is gone. metric_catalog() is core's own sources
  (the four monitor types and "steward", the app's open_fds/open_fds_pct,
  which were recorded but never offered) plus each loaded plugin's
  get_alert_metrics() export. A disabled plugin's sources leave the picker.
- traefik offers what it records: error_rate_4xx_pct and error_rate_5xx_pct.
  The old "error_rate" was never recorded, so a rule on it could not fire.
- tests/core/test_metric_catalog.py walks every MetricBatch.add /
  record_metric call (traefik's tuple loop included) and fails on a recorded
  metric the catalog lacks; it shows on a synthetic writer that it can fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/docker/__init__.py

@@ -62,3 +62,10 @@ async def mcp_overview(db) -> list[dict]:
 def get_nav() -> list[dict]:
     # Surfaces in the sidebar's Infrastructure group (fleet-wide container view).
     return [{"label": "Docker", "href": "/plugins/docker/"}]
+
+
+def get_alert_metrics() -> dict[str, list[str]]:
+    """What ingest.py records per container (resource "<host>/<container>"):
+    restart_count is cumulative (alert on crash-looping); is_healthy is 1.0 /
+    0.0 from the container's HEALTHCHECK, only for containers that define one."""
+    return {"docker": ["cpu_pct", "mem_pct", "restart_count", "is_healthy"]}