feat(alerts): the host's temperature and throughput can drive alert rules (#5787)

temp_c_max was recorded on every ingest, but host_agent didn't offer it
to rules, so "host running hot" couldn't be watched. The catalog guard
didn't see the row(...) helper ingest builds its rows with. Now offered:
temp_c_max, temp_c per sensor, and network and disk throughput per host,
interface and disk. A new guard holds every literal row(...) name to the
catalog, except NOT_ALERTABLE, where each exclusion is given a reason.
temp_c_max reads in °C.

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

plugins/host_agent/__init__.py

@@ -78,7 +78,18 @@ def get_alert_metrics() -> dict[str, list[str]]:
     reboot_required is 1 when the host needs a reboot (agent ≥ 1.16.0):
     "security updates waiting" is updates_security > 0. maintenance_failed
     comes from the scheduler (#5768): 1 when the host's latest maintenance job
-    failed or timed out, 0 once one succeeds: "maintenance failed" is > 0."""
+    failed or timed out, 0 once one succeeds: "maintenance failed" is > 0.
+    temp_c_max is the host's hottest real thermometer, in °C (#5787); sensors
+    with nothing wired to them are left out (#5759, #5783): "running hot" is
+    > 85. temp_c is one sensor, resource "<host>:temp:<label>". net_rx_bps /
+    net_tx_bps and disk_read_bps / disk_write_bps are bytes per second, for
+    the whole host ("<host>") or one interface ("<host>:net:<iface>") or disk
+    ("<host>:diskio:<device>").
+
+    Recorded but not offered (NOT_ALERTABLE): disk_used_bytes and
+    disk_total_bytes, where disk_used_pct is the figure to watch (and docker
+    uses disk_total_bytes for something else), and mem_cached_bytes /
+    mem_buffers_bytes, parts of what mem_available_bytes already counts."""
     return {"host_agent": [
         "cpu_pct", "mem_used_pct", "mem_available_bytes", "swap_used_bytes",
         "disk_used_pct_worst", "disk_used_pct", "inodes_used_pct", "load_1m", "load_5m", "load_15m",
@@ -86,4 +97,11 @@ def get_alert_metrics() -> dict[str, list[str]]:
         "uptime_secs", "seconds_since_seen", "clock_off_secs", "mounts_stalled",
         "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",
     ]}
+
+
+# Metrics host_agent records that no rule is offered, each for the reason
+# get_alert_metrics gives; tests/core/test_metric_catalog.py holds every other
+# recorded name to the catalog.
+NOT_ALERTABLE = frozenset({"disk_used_bytes", "disk_total_bytes", "mem_cached_bytes", "mem_buffers_bytes"})