feat(host_agent): a dropped ingest says which step of the metric write it stalled in (#5149)

The first instrumented drop on dev read "dropped after 10.0s … (completed:
parse=0.1s)": the whole stall is inside record_metrics. It now marks its
three steps (flush of the rows, the rule and state reads, the per-sample
evaluation) on the caller's phases, and the drop line carries the request's
samples and metric row count.

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

tests/plugins/host_agent/test_ingest_timing.py

@@ -51,7 +51,8 @@ async def test_a_dropped_ingest_says_how_far_it_got(app, monkeypatch, caplog):
         phases = routes._Phases()
         clock.t += 0.2
         phases.mark("parse")
+        phases.size = "samples=21 metric_rows=840"
         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
+    assert "dropped after 10.1s for token=abcd1234… (completed: parse=0.2s) samples=21 metric_rows=840" in caplog.text