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/integration/test_prometheus_scrape.py

@@ -1,6 +1,8 @@
 """Integration: a Prometheus scrape writes each target's outcome to its row,
 its kept series to the metric tables (a counter as a rate), and its own
-is_up, which an alert rule can watch (#5668). Requires STEWARD_DATABASE_URL."""
+is_up, which an alert rule can watch (#5668). A series a good scrape no
+longer reports loses its newest value (#5689); a failed scrape keeps them.
+Requires STEWARD_DATABASE_URL."""
 from __future__ import annotations
 
 import asyncio
@@ -14,7 +16,7 @@ pytestmark = pytest.mark.integration
 
 TEXT = """\
 # TYPE fa_backup_target_state gauge
-fa_backup_target_state{target="Steward",state="late"} 0
+fa_backup_target_state{target="Steward",state="STATE"} 1
 # TYPE fa_requests_total counter
 fa_requests_total COUNT
 # TYPE noise gauge
@@ -57,13 +59,14 @@ def test_a_scrape_records_series_rates_and_a_down_target_fires_its_rule(app):
     tag = uuid.uuid4().hex[:8]
     up, down = f"arch-{tag}", f"gone-{tag}"
     counts = iter([1_000, 1_011])
+    states = iter(["late", "fresh"])
     tokens = []
 
     async def fake_scrape(url, token, verify_tls):
         tokens.append(token)
         if "gone" in url:
             raise ScrapeError("Couldn't connect: connection refused.")
-        return prometheus.parse(TEXT.replace("COUNT", str(next(counts))))
+        return prometheus.parse(TEXT.replace("COUNT", str(next(counts))).replace("STATE", next(states)))
 
     async def latest(name_prefix):
         async with app.db_sessionmaker() as s:
@@ -117,7 +120,10 @@ def test_a_scrape_records_series_rates_and_a_down_target_fires_its_rule(app):
 
     first, second, rows, state, down_latest = asyncio.run(go())
     assert "scrape-token" in tokens
-    assert first[(f"{up}:target=Steward,state=late", "fa_backup_target_state")] == 0.0
+    assert first[(f"{up}:target=Steward,state=late", "fa_backup_target_state")] == 1.0
+    # The second scrape no longer reports "late": its newest value is forgotten.
+    assert (f"{up}:target=Steward,state=late", "fa_backup_target_state") not in second
+    assert second[(f"{up}:target=Steward,state=fresh", "fa_backup_target_state")] == 1.0
     assert (up, "fa_requests_per_sec") not in first                  # a counter needs two scrapes
     assert first[(up, "is_up")] == 1.0 and first[(up, "series_kept")] == 2.0
     assert not any(m == "noise" for _, m in first)                   # not in the keep list