feat(prometheus): scrape any /metrics endpoint into Steward's metrics, with pages (#5668, #5669)

A bundled prometheus plugin, on by default like the other protocol plugins.
Targets are rows edited on its page: address, bearer token (encrypted),
interval, keep/leave-out name patterns and a series cap. Steward does the
filtering; the endpoint serves everything (Archives' ruling).

- A 5 s tick scrapes due targets at once, each within a 30 s deadline, and
  writes the outcome to its row and its series through record_metrics, so
  alert rules can watch any of them.
- Gauges are stored as they are; counters (and a histogram's or summary's
  _sum/_count) as <name>_per_sec rates from the second scrape. Buckets and
  quantiles are skipped. A labelled series is stored under
  "<target>:k=v,…". Each target also records is_up, scrape_ms, series_kept
  and series_capped.
- Pages: targets not answering first, with why; a target's series grouped
  by metric with their newest values; a chart per series; the editor with
  Test, which scrapes once and says what would be stored.
- The counter-to-rate keeper moves to steward/core/counter_rates.py; SNMP's
  RateKeeper now scales its rate on top of it.
- metric_value reads Prometheus's _seconds as a span and _per_sec as a rate.
- The catalog guard treats an empty list as a dynamic source (snmp,
  prometheus), as the rule form does.

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

plugins/prometheus/__init__.py

@@ -0,0 +1,41 @@
+# plugins/prometheus/__init__.py
+from __future__ import annotations
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from quart import Quart
+
+_app: "Quart | None" = None
+
+
+def setup(app: "Quart") -> None:
+    global _app
+    _app = app
+    from .models import PrometheusTarget  # noqa: F401 — registers the table
+
+
+def get_scheduled_tasks() -> list:
+    from .scheduler import make_scrape_task
+    return [make_scrape_task(_app)]
+
+
+def get_blueprint():
+    from .routes import prometheus_bp
+    return prometheus_bp
+
+
+def get_nav() -> list[dict]:
+    # Surfaces in the sidebar's Infrastructure group (scraped endpoints).
+    return [{"label": "Prometheus", "href": "/plugins/prometheus/"}]
+
+
+def get_alert_metrics() -> dict[str, list[str]]:
+    """The metric names are whatever each endpoint exposes, so the list is
+    empty here and the rule form offers the names recorded so far."""
+    return {"prometheus": []}
+
+
+def get_audit_entities() -> dict:
+    """Audit rows about a target name it and link to its page."""
+    from .models import PrometheusTarget
+    return {"prometheus_target": (PrometheusTarget, "/plugins/prometheus/targets/{}")}