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/__init__.py

@@ -96,7 +96,9 @@ def get_alert_metrics() -> dict[str, list[str]]:
     average time a program waited on one (queueing included), nfs_rtt_ms the
     server's round trip, nfs_read_ms / nfs_write_ms the same for reads and
     writes alone, nfs_ops_per_sec how many, and nfs_retrans / nfs_timeouts the
-    retransmissions and major timeouts among them. nfs_exe_max_ms is the
+    retransmissions and major timeouts among them; nfs_tx_bps / nfs_rx_bps
+    (agent ≥ 1.25.0, #6115) are the bytes a second it sent and received, its
+    share of the host's network traffic. nfs_exe_max_ms is the
     host's slowest mount, for a covering rule: "NFS slow" is > 1000 for 3
     checks. A healthy share answers in a few ms.
 
@@ -125,7 +127,7 @@ def get_alert_metrics() -> dict[str, list[str]]:
         "updates_pending", "updates_security", "reboot_required", "maintenance_failed",
         "temp_c_max", "temp_c", "net_rx_bps", "net_tx_bps", "disk_read_bps", "disk_write_bps",
         "nfs_exe_max_ms", "nfs_exe_ms", "nfs_rtt_ms", "nfs_read_ms", "nfs_write_ms",
-        "nfs_ops_per_sec", "nfs_retrans", "nfs_timeouts",
+        "nfs_ops_per_sec", "nfs_retrans", "nfs_timeouts", "nfs_tx_bps", "nfs_rx_bps",
         *DRIVE_METRICS,
     ]}