feat(host_agent): say where a slow ingest spends its time, and how far a dropped one got (#5149)

Every host-agent ingest in the service logs ends at ~10 s with no status:
the agent gives up at its urlopen timeout (agent.py) before Steward answers.
Nothing says which part of ingest is slow, so this measures it:

- each phase (parse, metrics with alert evaluation, docker persist,
  commit) is timed, and a request past 2 s logs one line with the phases,
  sample count, metric rows, containers and log lines;
- a request the server drops when the agent hangs up (CancelledError) logs
  how long it ran and which phases completed, then re-raises.

The fix follows once the logs say which phase it is.

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

tests/plugins/host_agent/test_ingest_timing.py

@@ -0,0 +1,57 @@
+"""A slow ingest says where its time went, and one dropped when the agent hung
+up says how far it got (#5149)."""
+import logging
+
+from plugins.host_agent import routes
+
+
+class _Clock:
+    def __init__(self):
+        self.t = 100.0
+
+    def __call__(self):
+        return self.t
+
+
+async def test_a_slow_ingest_logs_its_phases(app, monkeypatch, caplog):
+    clock = _Clock()
+    monkeypatch.setattr(routes.time, "monotonic", clock)
+    async with app.app_context():
+        phases = routes._Phases()
+        clock.t += 0.1
+        phases.mark("parse")
+        clock.t += 8.0
+        phases.mark("metrics")
+        clock.t += 0.4
+        phases.mark("commit")
+        with caplog.at_level(logging.WARNING):
+            phases.report("web-1", 12, 480, 30, 900)
+    line = caplog.text
+    assert "slow 8.5s for host=web-1" in line
+    assert "parse=0.1s metrics=8.0s commit=0.4s" in line
+    assert "samples=12 metric_rows=480 containers=30 log_lines=900" in line
+
+
+async def test_a_quick_ingest_logs_nothing(app, monkeypatch, caplog):
+    clock = _Clock()
+    monkeypatch.setattr(routes.time, "monotonic", clock)
+    async with app.app_context():
+        phases = routes._Phases()
+        clock.t += 0.3
+        phases.mark("commit")
+        with caplog.at_level(logging.WARNING):
+            phases.report("web-1", 1, 40, 0, 0)
+    assert "host_agent ingest" not in caplog.text
+
+
+async def test_a_dropped_ingest_says_how_far_it_got(app, monkeypatch, caplog):
+    clock = _Clock()
+    monkeypatch.setattr(routes.time, "monotonic", clock)
+    async with app.app_context():
+        phases = routes._Phases()
+        clock.t += 0.2
+        phases.mark("parse")
+        clock.t += 9.9
+        with caplog.at_level(logging.WARNING):
+            phases.cancelled("abcd1234")
+    assert "dropped after 10.1s for token=abcd1234… (completed: parse=0.2s)" in caplog.text