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>

tests/plugins/prometheus/test_widget.py

@@ -0,0 +1,122 @@
+"""The Prometheus dashboard widget (#5691): one target drawn by one template,
+in widget shape, and the dashboard pieces it needs (settings read when the
+picker opens, a Details link naming the target)."""
+import asyncio
+from datetime import timedelta
+
+from plugins.prometheus import library
+from plugins.prometheus.routes import spark_heights
+from plugins.prometheus.template_format import check
+from plugins.prometheus.template_view import render
+from steward.dashboard.routes import detail_url, with_options
+from tests._templates import render_plugin_template
+from tests.plugins.prometheus.test_templates import NOW, SERIES, TEMPLATE, T, latest, states
+from tests.test_style_tokens import hex_colours
+
+
+def test_every_bundled_template_is_a_template():
+    entries = library.bundled()
+    assert [e.key for e in entries] == ["bundled:fabled_archives"]
+    for e in entries:
+        assert e.template is not None, [str(p) for p in e.problems]
+    assert entries[0].name == "Fabled Archives" and entries[0].template.expects == "fabledarchives_"
+
+
+def test_settings_read_when_the_picker_opens():
+    async def targets(db):
+        return [{"value": "t1", "label": "Archives"}]
+
+    async def none(db):
+        return []
+
+    widget = {"key": "w", "params": [
+        {"key": "target", "type": "select", "default": "", "options_from": targets},
+        {"key": "template", "type": "select", "default": "", "options_from": none, "empty": "No templates yet."},
+        {"key": "hours", "type": "select", "default": 6, "options": [{"value": 6, "label": "6h"}]}]}
+    filled = asyncio.run(with_options(None, widget))
+    assert filled["params"][0]["options"] == [{"value": "t1", "label": "Archives"}]
+    assert filled["params"][1]["options"] == []
+    assert filled["params"][2] is widget["params"][2]
+    assert "options" not in widget["params"][0]                     # the registry isn't changed
+    plain = {"key": "x", "params": []}
+    assert asyncio.run(with_options(None, plain)) is plain
+
+
+def test_details_names_the_target_or_falls_back_to_the_section():
+    defn = {"detail_url": "/plugins/prometheus/targets/{target}"}
+    assert detail_url(defn, {"target": "a b/c", "template": "x"}) == "/plugins/prometheus/targets/a%20b%2Fc"
+    assert detail_url(defn, {}) == "/plugins/prometheus/"
+    assert detail_url({"detail_url": "/monitors/"}, {"limit": 5}) == "/monitors/"
+
+
+def test_bars_scale_to_their_own_peak():
+    assert spark_heights([1.0, 2.0, 4.0]) == [25.0, 50.0, 100.0]
+    assert spark_heights([0.0, 0.0]) == [0.0, 0.0] and spark_heights([]) == []
+
+
+def _target(reach="up", ok_minutes=1):
+    at = NOW - timedelta(minutes=ok_minutes)
+    return {"id": "t1", "name": T, "reach": reach, "says": "", "ok_at": at, "scraped_at": NOW}
+
+
+async def _page(app, user=True, **ctx):
+    from quart import session
+    async with app.test_request_context("/plugins/prometheus/widget"):
+        if user:
+            session["user_id"] = "u1"
+            session["user_role"] = "viewer"
+        ctx.setdefault("spark_hours", 24)
+        page = await render_plugin_template(app, "prometheus", "prometheus/widget.html", **ctx)
+    return " ".join(page.split())
+
+
+async def test_the_widget_leads_with_the_verdict_then_problems_first(app):
+    view = render(check(TEMPLATE), T, latest(SERIES), NOW,
+                  history={(T, "fa_disk_free_bytes"): [50.0, 100.0]})
+    page = await _page(app, problem="", view=view, target=_target())
+    assert page.index("3 need a look. Backup targets: Photos is late.") < page.index("Backup targets</div>")
+    assert 'class="w-verdict tone-warn"' in page
+    # Photos (late) before Steward (fresh) before Old (disabled, quiet).
+    targets = page[page.index("Backup targets</div>"):page.index("Storage</div>")]
+    assert targets.index("Photos") < targets.index("Steward") < targets.index("Old")
+    assert '<span class="w-sub tone-warn">is late</span>' in page
+    assert "<th>Now</th>" in page and '<th class="w-wide-only">Last backup</th>' in page
+    assert "16.2 TiB of 41.8 TiB" in page and 'class="meter-fill meter-ok" style="width:38.7%"' in page
+    assert '<span class="w-bars w-wide-only"' in page and 'style="height:50.0%"' in page
+    assert "<th>files</th><th>trash</th>" in page and "437.7 GiB" in page
+    assert "version 2026.10.12.1717 · commit 3605d0a" in page
+    assert '<a href="/plugins/prometheus/targets/t1">Archives</a>' in page
+    assert not hex_colours(page), hex_colours(page)
+
+
+async def test_the_healthy_rest_folds_after_six(app):
+    many = {**SERIES}
+    for i in range(9):
+        many.update(states(f"target=T{i},target_id=x{i}", "fresh", ["fresh", "late"]))
+    view = render(check(TEMPLATE), T, latest(many), NOW)
+    page = await _page(app, problem="", view=view, target=_target())
+    # 1 late, 10 fresh, 1 quiet in Backup targets: the late one, six of the eleven others.
+    assert "+5 more healthy" in page
+
+
+async def test_a_target_not_answering_says_so_first(app):
+    view = render(check(TEMPLATE), T, latest(SERIES), NOW)
+    page = await _page(app, problem="", view=view, target=_target("down", ok_minutes=90))
+    assert '<div class="w-verdict tone-crit">Archives isn\'t answering; these values are from' in page
+    assert page.index("isn't answering") < page.index("3 need a look")
+
+
+async def test_a_widget_that_cant_draw_says_why(app):
+    page = await _page(app, problem="The target this widget showed has been deleted.", view=None, target=None)
+    assert '<div class="w-verdict tone-warn">The target this widget showed has been deleted.</div>' in page
+
+
+async def test_a_shared_dashboard_gets_no_links(app):
+    view = render(check(TEMPLATE), T, latest(SERIES), NOW)
+    page = await _page(app, user=False, problem="", view=view, target=_target())
+    assert "/plugins/prometheus/targets/t1" not in page and "Archives, scraped" in page
+
+
+def test_the_widget_names_itself_after_its_template():
+    assert asyncio.run(library.default_title(None, {"template": "bundled:fabled_archives"})) == "Fabled Archives"
+    assert asyncio.run(library.default_title(None, {"template": "bundled:nope"})) is None