fix(host_agent): every Docker read is bounded, and the agent reports its own footprint (#5764)

#5764 "The agent reads a container's whole log backlog into memory, on its
main loop, with no overall deadline". Docker03's 1.13.0 agent went silent
for a day at 2.1 GB and 46 h of CPU, then ignored SIGTERM.

- One shared reader, _docker_get: each answer has an overall deadline and a
  byte cap, not only a per-recv timeout. A trickling or endless body is cut
  off (15 s / 32 MiB for JSON, 5 s / 4 MiB for a log read).
- Container logs: since the cursor but only the newest 2,000 lines, all
  within 10 s. A note says when older lines were skipped, and a container
  whose answer is too big skips ahead to now instead of re-reading its
  backlog every interval.
- The agent reports its own RSS and CPU; Steward records agent_rss_bytes
  and agent_cpu_pct, so a runaway agent shows before it goes silent.

Agent 1.15.0.

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

plugins/host_agent/routes.py

@@ -149,6 +149,14 @@ def _expand_sample_to_metrics(
     if sample.get("uptime_secs") is not None:
         row("uptime_secs", host_name, sample["uptime_secs"])
 
+    # The agent's own footprint (#5764), so a runaway agent shows before it
+    # goes silent.
+    me = sample.get("agent") or {}
+    if me.get("rss_bytes") is not None:
+        row("agent_rss_bytes", host_name, me["rss_bytes"])
+    if me.get("cpu_pct") is not None:
+        row("agent_cpu_pct", host_name, me["cpu_pct"])
+
     # The fullest disk is the fullest by space or by inodes (#5745): out of
     # inodes, every file create fails with space to spare, so the disk rules
     # and the overview, which read disk_used_pct_worst, see either.