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_pages.py

@@ -60,7 +60,11 @@ def test_series_are_grouped_by_metric_with_their_labels_and_units():
     assert [g["metric"] for g in groups] == ["fa_disk_free_bytes", "fa_last_backup_timestamp_seconds", "fa_state"]
     assert groups[0]["rows"][0]["labels"] == "" and groups[0]["rows"][0]["text"] == "2.0 GiB"
     assert groups[1]["rows"][0]["moment"] == datetime.fromtimestamp(stamp, timezone.utc)
-    assert [r["labels"] for r in groups[2]["rows"]] == ["target=S,state=fresh", "target=S,state=late"]
+    # A state set reads as one row naming the current state.
+    assert groups[2]["states"] and not groups[0]["states"]
+    assert [(r["labels"], r["text"], r["states"]) for r in groups[2]["rows"]] == [
+        ("target=S", "fresh", ["fresh", "late"])]
+    assert groups[2]["rows"][0]["resource"] == "A:target=S,state=fresh"
     assert shown_value("x_timestamp_seconds", 0) == {"text": "never", "moment": None}
     assert shown_value("jobs_per_sec", 0.25)["text"] == "0.25/s"
     assert shown_value("job_seconds", 90)["text"] == "1m 30s"
@@ -69,9 +73,9 @@ def test_series_are_grouped_by_metric_with_their_labels_and_units():
 
 async def test_the_target_page_leads_with_its_state_then_its_series(app):
     from quart import session
-    groups = [{"metric": "fa_state", "rows": [
-        {"labels": "target=S,state=late", "resource": "archives:target=S,state=late", "at": None,
-         "text": "0", "moment": None}]}]
+    groups = [{"metric": "fa_state", "states": True, "rows": [
+        {"labels": "target=S", "resource": "archives:target=S,state=late", "moment": None,
+         "text": "late", "states": ["fresh", "late"]}]}]
     async with app.test_request_context("/plugins/prometheus/targets/id-archives"):
         session["user_role"] = "viewer"
         page = await render_plugin_template(app, "prometheus", "prometheus/target.html",
@@ -81,6 +85,7 @@ async def test_the_target_page_leads_with_its_state_then_its_series(app):
     assert page.index("Answering") < page.index("Series stored") < page.index("fa_state")
     assert "3 series weren't stored" in page and "Keeping fabledarchives_*" in page
     assert "resource=archives%3Atarget%3DS%2Cstate%3Dlate" in page and "metric=fa_state" in page
+    assert 'title="One of: fresh, late">late</a>' in page and "a state set" in page
     assert 'href="/plugins/prometheus/">Prometheus</a>' in page          # its breadcrumb
     assert "/edit" not in page