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/homeassistant/templates/homeassistant/index.html

@@ -73,6 +73,22 @@
   {% if others %}<p class="field-hint" style="margin-bottom:0;">Waiting, for Home Assistant to install: {% for u in others %}{{ u.title }}{{ ", " if not loop.last }}{% endfor %}. Steward installs Core, OS and add-on updates only.</p>{% endif %}
 </section>
 
+{% if metrics %}
+<section class="card" aria-labelledby="ha-metrics">
+  <h2 id="ha-metrics" class="section-title">Metrics</h2>
+  {% if metrics.uses_sign_in %}
+  <p class="field-hint" style="margin-bottom:0;">{{ status_badge(metrics.reach) }} Read with this sign-in into the target <a href="/plugins/prometheus/targets/{{ metrics.id }}">{{ metrics.name }}</a>: {{ metrics.says }}{% if metrics.reach == 'up' %} {{ metrics.kept }} series kept.{% endif %}</p>
+  {% elif metrics.id %}
+  <p class="field-hint">The target <a href="/plugins/prometheus/targets/{{ metrics.id }}">{{ metrics.name }}</a> reads these with a token of its own: {{ metrics.says }}</p>
+  {% if admin %}<p class="field-hint">Use this sign-in instead, so one Home Assistant user covers everything. The target keeps its name, filters and history, and forgets its token.</p>
+  <form method="post" action="/plugins/homeassistant/metrics">{{ csrf_field() }}<button class="btn btn-sm" type="submit">Use this sign-in</button></form>{% endif %}
+  {% else %}
+  <p class="field-hint">Home Assistant's entity readings can be kept as metrics too, for charts and alert rules, with this sign-in and nothing else to set up.</p>
+  {% if admin %}<form method="post" action="/plugins/homeassistant/metrics">{{ csrf_field() }}<button class="btn btn-sm" type="submit">Read its metrics</button></form>{% endif %}
+  {% endif %}
+</section>
+{% endif %}
+
 {% if runs %}
 <section class="card" aria-labelledby="ha-runs">
   <h2 id="ha-runs" class="section-title">Updates run from Steward</h2>