feat(host_agent): NFS delay per mount, read from /proc/self/mountstats (#5996)

Each sample carries each NFS mount's requests since the last one: how many
a second, their round trip and execute time (READ and WRITE on their own),
and how many were sent again or hit a major timeout. mountstats is the
kernel's own count and never touches the share, so it reads while one is
stuck. Steward stores them per mount and the slowest on the host as
nfs_exe_max_ms, for an "NFS slow" rule and the host page's NFS chart beside
mounts_stalled; each mount's bar says what a request took.

Agent 1.22.0 (which also carries the backlog order from 41fc807).

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

plugins/host_agent/__init__.py

@@ -91,6 +91,15 @@ def get_alert_metrics() -> dict[str, list[str]]:
     the whole host ("<host>") or one interface ("<host>:net:<iface>") or disk
     ("<host>:diskio:<device>").
 
+    NFS delay (agent ≥ 1.22.0, #5996), per NFS mount on "<host>:<mount>",
+    over the requests it finished since the last sample: nfs_exe_ms is the
+    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
+    host's slowest mount, for a covering rule: "NFS slow" is > 1000 for 3
+    checks. A healthy share answers in a few ms.
+
     Drive health (agent ≥ 1.20.0, #5789), read hourly with smartctl by the
     root helper and judged in drives.py, per drive on "<host>:drive:<device>":
     drive_failing is 1 when the drive fails its own self-assessment, raises
@@ -115,6 +124,8 @@ def get_alert_metrics() -> dict[str, list[str]]:
         "agent_rss_bytes", "agent_cpu_pct",
         "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",
         *DRIVE_METRICS,
     ]}