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>

steward/core/counter_rates.py

@@ -0,0 +1,34 @@
+"""A counter's previous sample, to turn the next one into a per-second rate.
+
+Scrapers that read monotonic counters (SNMP octets, Prometheus *_total)
+store the rate, not the running total. Keys are tuples whose first element
+is the owner (a device, a target), so forgetting an owner drops all of its
+counters at once.
+"""
+from __future__ import annotations
+
+from datetime import datetime
+
+
+class CounterRates:
+    """In memory: after a restart the first sample of each counter gives no
+    rate and the second does. A sample lower than the one before (the source
+    restarted, or a 32-bit counter wrapped) gives no rate rather than a huge
+    negative or wrapped one."""
+
+    def __init__(self) -> None:
+        self._last: dict[tuple, tuple[float, datetime]] = {}
+
+    def rate(self, key: tuple, value: float, at: datetime) -> float | None:
+        prev = self._last.get(key)
+        self._last[key] = (value, at)
+        if prev is None:
+            return None
+        seconds = (at - prev[1]).total_seconds()
+        if seconds <= 0 or value < prev[0]:
+            return None
+        return (value - prev[0]) / seconds
+
+    def forget(self, owner) -> None:
+        for key in [k for k in self._last if k[0] == owner]:
+            del self._last[key]