feat(prometheus): the Prometheus dashboard widget, target + template (#5691)

#5691 "Generic Prometheus dashboard widget: pick a target and a template",
step 3 of milestone 508 "Prometheus widget templates".

- /plugins/prometheus/widget draws one target with one template in widget
  shape: the verdict (led by "isn't answering" when the target is down),
  then each block with problems first, figures beside a share bar or the
  last 24h of bars, and the healthy rest folding to "+N more healthy" past
  six. A missing target or template, or one that no longer checks, says so.
- library.py: bundled templates from plugins/prometheus/widgets/*.yaml,
  the picker's target and template lists, and the default title.
- widgets/fabled_archives.yaml: the first bundled template, against the
  series Archives reports (refined in #5693).
- Dashboard: a widget setting may take `options_from` (read when the picker
  opens) with an `empty` sentence; `default_title`; a Details link may name
  settings ("/plugins/prometheus/targets/{target}"). Prometheus is grouped
  with the integrations.

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

plugins/prometheus/templates/prometheus/widget.html

@@ -0,0 +1,70 @@
+{# prometheus/widget.html — the Prometheus dashboard widget (#5691): one
+   target drawn by one template (template_view.render). A verdict naming the
+   worst thing, then each block: its problems first with why, figures in their
+   units beside a bar or the last day's bars, and the healthy rest packed after,
+   past REST rows as "+N more healthy". #}
+{% from "_macros.html" import status_dot %}
+{% set REST = 6 %}
+{% set dot = {"crit": "crit", "warn": "warn", "ok": "ok", "dim": "paused"} %}
+{% set tone = {"crit": "tone-crit", "warn": "tone-warn", "ok": "tone-ok", "dim": "tone-dim"} %}
+{% macro shown(v) -%}
+  {%- if v.moment %}{{ v.moment|ago }}{% elif v.text %}{{ v.text }}{% else %}—{% endif -%}
+{%- endmacro %}
+{% if problem %}
+<div class="w-verdict tone-warn">{{ problem }}</div>
+{% else %}
+{% set down = target.reach in ("down", "stale") %}
+<div class="w-verdict {{ 'tone-crit' if target.reach == 'down' else tone[view.tone] }}">
+  {%- if target.reach == "down" %}{{ target.name }} isn't answering{% elif down %}{{ target.name }} isn't being scraped{% endif %}
+  {%- if down %}{% if target.ok_at %}; these values are from {{ target.ok_at|ago }}{% endif %}. {% endif -%}
+  {{ view.verdict }}
+</div>
+{% for b in view.blocks %}
+<div class="w-section">{{ b.title }}</div>
+{% if b.note %}
+<div class="w-note tone-warn">{{ b.note }}</div>
+{% elif b.kind == "info" %}
+<div class="w-note">{% for r in b.rows %}{{ r.name }}{% if not loop.last %}; {% endif %}{% endfor %}</div>
+{% elif b.kind == "table" %}
+<table class="w-table">
+  <thead><tr><th></th>{% for c in b.columns %}<th>{{ c }}</th>{% endfor %}</tr></thead>
+  <tbody>
+  {% for r in b.rows %}
+  <tr><td class="w-name" title="{{ r.name }}">{{ r.name }}</td>{% for c in r.cells %}<td>{{ shown(c) }}</td>{% endfor %}</tr>
+  {% endfor %}
+  </tbody>
+</table>
+{% else %}
+{% set problems = b.rows | selectattr("tone", "in", ["crit", "warn"]) | list %}
+{% set rest = b.rows | rejectattr("tone", "in", ["crit", "warn"]) | list %}
+{% set listed = problems + rest[:REST] %}
+<table class="w-table">
+  {% if b.columns %}<thead><tr><th></th><th>Now</th>{% for c in b.columns %}<th class="w-wide-only">{{ c }}</th>{% endfor %}</tr></thead>{% endif %}
+  <tbody>
+  {% for r in listed %}
+  <tr>
+    <td class="w-name" title="{{ r.name }}">
+      {{- status_dot(dot[r.tone]) -}}
+      {{ r.name }}
+      {%- if r.why %}<span class="w-sub {{ tone[r.tone] }}">{{ r.why }}</span>{% endif %}
+    </td>
+    <td class="{{ tone[r.tone] if r.tone in ('crit', 'warn') }}">
+      {%- if r.history %}<span class="w-bars w-wide-only" aria-hidden="true" title="The last {{ spark_hours }}h, scaled to its own peak">
+        {%- for h in r.history %}<i style="height:{{ [h, 4]|max }}%"></i>{% endfor -%}
+      </span>{% endif -%}
+      {{ shown(r) }}
+      {%- if r.bar is not none %}<div class="meter w-wide-only" title="{{ (r.bar * 100)|round|int }}%"><div class="meter-fill meter-{{ r.tone if r.tone in ('crit', 'warn') else 'ok' }}" style="width:{{ (r.bar * 100)|round(1) }}%"></div></div>{% endif -%}
+    </td>
+    {% for c in r.cells %}<td class="w-wide-only">{{ shown(c) }}</td>{% endfor %}
+  </tr>
+  {% endfor %}
+  </tbody>
+</table>
+{% if rest|length > REST %}<div class="w-more">+{{ rest|length - REST }} more healthy</div>{% endif %}
+{% endif %}
+{% endfor %}
+<div class="w-note">
+  {%- if session.user_id %}<a href="/plugins/prometheus/targets/{{ target.id }}">{{ target.name }}</a>{% else %}{{ target.name }}{% endif %}
+  {%- if target.scraped_at %}, scraped {{ target.scraped_at|ago }}{% endif %}.
+</div>
+{% endif %}