feat(traffic): each container's network rate, and each NFS mount's bytes, so a host's traffic has a sender (#6115)

Docker02's outbound traffic tops out the internet connection and Steward
couldn't say who sends it: a container's counters were kept only as a
running total, and NFS traffic belongs to no container.

- Docker ingest turns each running container's cumulative network counters
  into net_rx_bps / net_tx_bps on "<host>/<container>" (net_rates), counting
  from the stored row so a restart of Steward loses nothing; a container
  new, recreated or restarted has no rate until its next snapshot.
- Agent 1.25.0 reads the bytes sent and received per NFS mount from
  mountstats (already parsed for the delays) as tx_bps / rx_bps; Steward
  stores them as nfs_tx_bps / nfs_rx_bps on "<host>:<mount>".

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

plugins/host_agent/routes.py

@@ -292,7 +292,8 @@ def _expand_sample_to_metrics(
         row("mounts_stalled", host_name, stalled)
 
     # NFS delay per mount (agent ≥ 1.22.0, #5996): what its requests took
-    # since the last sample. The host's slowest mount is its own metric, for
+    # since the last sample, and (agent ≥ 1.25.0, #6115) the bytes a second
+    # it sent and received. The host's slowest mount is its own metric, for
     # a rule; recorded only when some mount finished a request, since a mount
     # with none, idle or stuck, has no delay to give (mounts_stalled covers
     # stuck).
@@ -302,7 +303,8 @@ def _expand_sample_to_metrics(
         for key, metric in (("ops_per_sec", "nfs_ops_per_sec"), ("rtt_ms", "nfs_rtt_ms"),
                             ("exe_ms", "nfs_exe_ms"), ("read_ms", "nfs_read_ms"),
                             ("write_ms", "nfs_write_ms"), ("retrans", "nfs_retrans"),
-                            ("timeouts", "nfs_timeouts")):
+                            ("timeouts", "nfs_timeouts"), ("tx_bps", "nfs_tx_bps"),
+                            ("rx_bps", "nfs_rx_bps")):
             if isinstance(nfs.get(key), (int, float)):
                 row(metric, mount_res, nfs[key])
         if isinstance(nfs.get("exe_ms"), (int, float)):