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/snmp/__init__.py

@@ -26,3 +26,9 @@ def get_blueprint():
 def get_nav() -> list[dict]:
     # Surfaces in the sidebar's Infrastructure group (SNMP device readings).
     return [{"label": "SNMP", "href": "/plugins/snmp/"}]
+
+
+def get_alert_metrics() -> dict[str, list[str]]:
+    """SNMP metric names are the OID labels each device is configured with, so
+    the list is empty here and the rule form offers the labels recorded so far."""
+    return {"snmp": []}