feat(prometheus): fold state sets on the target page; forget series an endpoint drops (#5689)

#5689 "Prometheus target page: fold state sets, show epoch times, size
bytes", step 1 of milestone 508 "Prometheus widget templates".

- A state set (name ending in _state, a state label, every series 0 or 1;
  or the OpenMetrics form, the label named after the metric) reads as one
  row per thing it describes, naming the current state, the others on
  hover. fabledarchives_jobs{state} is a count and stays as it is.
- label_pairs / state_label / fold_states in service.py, for the widget
  templates to reuse.
- After a good scrape, series it no longer has lose their newest value
  (forget_gone): Archives' old build_info row read as current beside the
  new one. History stays; a failed scrape forgets nothing; alert rules keep
  their own last-sample time.
- Epoch times and _bytes sizes were already read by shown_value /
  metric_value.

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

plugins/prometheus/routes.py

@@ -20,8 +20,8 @@ from steward.models.users import UserRole
 
 from .models import PrometheusTarget
 from .scraper import ScrapeError, scrape
-from .service import (OWN_METRICS, SOURCE, FormError, form_values, map_families, parse_form,
-                      patterns, target_state)
+from .service import (OWN_METRICS, SOURCE, FormError, fold_states, form_values, map_families,
+                      parse_form, patterns, target_state)
 
 prometheus_bp = Blueprint("prometheus", __name__, template_folder="templates")
 
@@ -256,7 +256,9 @@ async def test_target():
 
 def group_series(target_name: str, latest: list) -> list[dict]:
     """The target's stored series grouped by metric, each with its rows:
-    the labels that tell them apart, and the newest value."""
+    the labels that tell them apart, and the newest value. A state set is
+    folded to one row per thing it describes, naming the current state
+    (`states` set on the group)."""
     groups: dict[str, list] = {}
     for row in latest:
         if row.resource_name == target_name and row.metric_name in OWN_METRICS:
@@ -264,9 +266,18 @@ def group_series(target_name: str, latest: list) -> list[dict]:
         labels = row.resource_name[len(target_name) + 1:] if row.resource_name != target_name else ""
         groups.setdefault(row.metric_name, []).append({
             "labels": labels, "resource": row.resource_name, "at": row.recorded_at,
-            **shown_value(row.metric_name, row.value)})
-    return [{"metric": m, "rows": sorted(rows, key=lambda r: r["labels"])}
-            for m, rows in sorted(groups.items())]
+            "value": row.value, **shown_value(row.metric_name, row.value)})
+    out = []
+    for metric, rows in sorted(groups.items()):
+        folded = fold_states(metric, rows)
+        if folded is not None:
+            out.append({"metric": metric, "states": True, "rows": [
+                {"labels": f["labels"], "resource": f["resource"], "moment": None,
+                 "text": f["state"] or "none", "states": f["states"]} for f in folded]})
+        else:
+            out.append({"metric": metric, "states": False,
+                        "rows": sorted(rows, key=lambda r: r["labels"])})
+    return out
 
 
 @prometheus_bp.get("/targets/<target_id>")