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>

tests/plugins/prometheus/test_service.py

@@ -8,8 +8,8 @@ import pytest
 from plugins.prometheus.models import PrometheusTarget
 from plugins.prometheus.scheduler import is_due
 from plugins.prometheus.scraper import ScrapeError, scrape
-from plugins.prometheus.service import (FormError, map_families, parse_form, patterns,
-                                        rate_name, resource_for, target_state, wanted)
+from plugins.prometheus.service import (FormError, fold_states, label_pairs, map_families, parse_form,
+                                        patterns, rate_name, resource_for, target_state, wanted)
 from steward.core import prometheus
 from steward.core.counter_rates import CounterRates
 
@@ -75,6 +75,38 @@ def test_names():
     assert patterns("a_*, b_*\n\n a_* \nc") == ["a_*", "b_*", "c"]
 
 
+def test_labels_read_back_as_stored_even_with_a_comma_in_a_value():
+    labels = resource_for("A", (("job", "Nightly"), ("schedule", "0 3 * * 1,3,5"), ("state", "ok")))
+    assert label_pairs(labels[2:]) == [("job", "Nightly"), ("schedule", "0 3 * * 1,3,5"), ("state", "ok")]
+    assert label_pairs("") == []
+
+
+def _rows(metric_rows):
+    return [{"labels": labels, "value": value, "resource": f"A:{labels}"} for labels, value in metric_rows]
+
+
+def test_a_state_set_folds_to_its_current_state_per_thing():
+    rows = _rows([("target=S,state=late", 0), ("target=S,state=fresh", 1), ("target=S,state=missing", 0),
+                  ("target=T,state=fresh", 0), ("target=T,state=late", 1), ("target=T,state=missing", 1),
+                  ("target=U,state=fresh", 0), ("target=U,state=late", 0)])
+    folded = fold_states("fa_backup_target_state", rows)
+    assert [(f["labels"], f["state"]) for f in folded] == [
+        ("target=S", "fresh"), ("target=T", "late, missing"), ("target=U", "")]
+    assert folded[0]["states"] == ["fresh", "late", "missing"]
+    assert folded[0]["resource"] == "A:target=S,state=fresh"            # the current one's chart
+    assert folded[2]["resource"] == "A:target=U,state=fresh"            # none current: the first
+    # OpenMetrics names a stateset's label after the metric itself.
+    assert fold_states("fa_mode", _rows([("fa_mode=a", 1), ("fa_mode=b", 0)]))[0]["state"] == "a"
+
+
+def test_only_a_state_set_is_folded():
+    # A count split by a state label reads 0 or 1 often enough; its name says it is a count.
+    assert fold_states("fa_jobs", _rows([("kind=scan,state=waiting", 1), ("kind=scan,state=running", 0)])) is None
+    assert fold_states("fa_state", _rows([("state=a", 1), ("state=b", 2)])) is None      # not 0/1
+    assert fold_states("fa_state", _rows([("state=a", 1), ("target=S", 0)])) is None     # a row without one
+    assert fold_states("fa_state", []) is None
+
+
 def test_a_counter_becomes_a_rate_and_a_reset_is_skipped():
     rates = CounterRates()
     t0 = datetime(2026, 10, 12, 12, 0, tzinfo=timezone.utc)