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>

steward/core/prometheus.py

@@ -21,6 +21,12 @@ _LABEL_NAME = re.compile(r"[a-zA-Z_][a-zA-Z0-9_]*")
 
 TYPES = ("counter", "gauge", "histogram", "summary", "untyped")
 
+
+class TokenUnavailable(Exception):
+    """A plugin that lends its sign-in to a scrape has no token to give, said to
+    the person. Nothing is sent to the endpoint."""
+
+
 # Suffixes a histogram or summary's samples carry beyond the family name.
 _CHILD_SUFFIXES = {
     "counter": ("_total",),        # OpenMetrics declares a counter without it