feat(homeassistant): its page offers the metrics scrape under the same sign-in, so one user covers everything (#6744)

A Prometheus target's token_from names the plugin that supplies its token at each
scrape. Read its metrics makes the target; Use this sign-in moves one at that address
across, keeping its name, filters and history. A refusal stops the connection, so one
Try again or sign-in starts everything, and the sign-in is only sent to its own address.
A 404 says how to turn on Home Assistant's Prometheus integration, as Steward doesn't
change Home Assistant. Refused also joins the volatile columns a backup check ignores.

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

plugins/prometheus/scraper.py

@@ -23,6 +23,10 @@ class TokenRefused(ScrapeError):
     Home Assistant, for one, bans an address after a few refusals."""
 
 
+class NoMetricsHere(ScrapeError):
+    """The address answered 404: nothing is serving metrics there."""
+
+
 async def scrape(url: str, token: str = "", verify_tls: bool = True,
                  transport: httpx.AsyncBaseTransport | None = None) -> dict[str, prometheus.Family]:
     """The endpoint's families. `transport` stands in for the network in tests."""
@@ -36,6 +40,8 @@ async def scrape(url: str, token: str = "", verify_tls: bool = True,
                 if response.status_code in (401, 403):
                     raise TokenRefused(f"Answered {response.status_code}: the token was refused. "
                                        "Check the token, or make a new one on the service.")
+                if response.status_code == 404:
+                    raise NoMetricsHere(f"Answered 404 {response.reason_phrase}, not metrics.")
                 if response.status_code != 200:
                     raise ScrapeError(f"Answered {response.status_code} {response.reason_phrase}, not metrics.")
                 body = bytearray()