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>

plugins/host_agent/routes.py

@@ -250,8 +250,8 @@ async def ingest():
 
             # Through the alert pipeline, so rules on host metrics can fire.
             from steward.core.alerts import record_metrics
-            await record_metrics(session, metric_rows)
-            phases.mark("metrics")
+            phases.size = f"samples={accepted} metric_rows={len(metric_rows)}"
+            await record_metrics(session, metric_rows, phases)
 
             # Hand host-scoped container data to the docker plugin if it's enabled
             # (opportunistic synergy via the capability registry — no hard import,
@@ -331,6 +331,7 @@ class _Phases:
     def __init__(self):
         self.start = self.last = time.monotonic()
         self.took: dict[str, float] = {}
+        self.size = ""
 
     def mark(self, name: str) -> None:
         now = time.monotonic()
@@ -339,9 +340,9 @@ class _Phases:
 
     def cancelled(self, who: str) -> None:
         current_app.logger.warning(
-            "host_agent ingest: dropped after %.1fs for token=%s… (completed: %s)",
+            "host_agent ingest: dropped after %.1fs for token=%s… (completed: %s) %s",
             time.monotonic() - self.start, who,
-            " ".join(f"{k}={v:.1f}s" for k, v in self.took.items()) or "nothing")
+            " ".join(f"{k}={v:.1f}s" for k, v in self.took.items()) or "nothing", self.size)
 
     def report(self, host: str, samples: int, metrics: int, containers: int, log_lines: int) -> None:
         total = time.monotonic() - self.start