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>

tests/plugins/docker/test_net_rates.py

@@ -0,0 +1,37 @@
+"""net_rates: a container's bytes a second, from its cumulative network
+counters against the last ones seen for it (#6115). Pure; no DB."""
+from datetime import datetime, timedelta, timezone
+
+from plugins.docker.ingest import net_rates
+
+T0 = datetime(2026, 10, 19, 18, 0, tzinfo=timezone.utc)
+
+
+def _c(name, rx, tx, cid="abc", status="running"):
+    return {"name": name, "container_id": cid, "status": status,
+            "net_rx_bytes": rx, "net_tx_bytes": tx}
+
+
+def test_rates_count_from_the_stored_row_and_then_from_each_snapshot():
+    before = {"web": (T0, "abc", 1_000, 2_000)}
+    snaps = [(T0 + timedelta(seconds=10), [_c("web", 11_000, 52_000)]),
+             (T0 + timedelta(seconds=20), [_c("web", 11_000, 62_000)])]
+    assert net_rates(before, snaps) == [
+        (T0 + timedelta(seconds=10), "web", 1_000.0, 5_000.0),
+        (T0 + timedelta(seconds=20), "web", 0.0, 1_000.0),
+    ]
+
+
+def test_no_rate_for_a_new_recreated_or_restarted_container_until_its_next_snapshot():
+    before = {"db": (T0, "old", 100, 100), "app": (T0, "abc", 5_000, 5_000)}
+    t1, t2 = T0 + timedelta(seconds=10), T0 + timedelta(seconds=20)
+    snaps = [(t1, [_c("new", 10, 10), _c("db", 200, 200, cid="fresh"), _c("app", 10, 10)]),
+             (t2, [_c("new", 110, 210), _c("db", 300, 300, cid="fresh"), _c("app", 20, 30)])]
+    assert net_rates(before, snaps) == [
+        (t2, "new", 10.0, 20.0), (t2, "db", 10.0, 10.0), (t2, "app", 1.0, 2.0)]
+
+
+def test_stopped_containers_and_missing_counters_are_skipped():
+    before = {"web": (T0, "abc", 0, 0), "old": (T0, "abc", 0, 0)}
+    snaps = [(T0 + timedelta(seconds=10), [_c("web", None, None), _c("old", 9, 9, status="exited")])]
+    assert net_rates(before, snaps) == []