feat(mcp): SNMP devices and Prometheus targets: their state, readings and series (#5840)

The snmp plugin registers list_snmp_devices and get_snmp_device; the
prometheus plugin registers list_prometheus_targets and
get_prometheus_target. Both are viewer-level, read what the poller and
scraper stored, and exist only while their plugin is enabled.
get_overview names a device or target whose last poll or scrape failed;
stale, pending and off are left out.

A device's community and v3 keys and a target's token are never
returned, and a password in a target URL is masked.

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

docs/core/mcp.md

@@ -82,6 +82,8 @@ Each role includes the tools of the roles above it in this list.
 | Host agent (built in, always on) | `list_agents`, `list_maintenance_plans` (each plan's group, weekly slot, next run and latest run: how far it got, who it skipped and why), `get_maintenance_job` (a host's update jobs or one job: state, packages updated, whether it rebooted, why it failed) |
 | Traefik plugin | `list_routers` (each router graded as the Traefik page grades it, with the status codes behind a problem, what they mean and the swarm service behind it), `get_router` (one router's history, codes, normal and anomalies), `list_certificates` (served certificates, days left, the routers they cover), `get_traffic` (the access log's clients, countries and routers, and Traefik's own figures) |
 | UniFi plugin | `get_network` (the WAN, clients, Wi-Fi networks, speed tests and devices by status), `list_devices`, `list_clients` (everyone seen, or the newest poll's top talkers), `list_network_events`, `list_alarms`, `get_network_config` (networks and VLANs, port forwards, firewall rules) |
+| SNMP plugin | `list_snmp_devices` (each device's state, newest readings in their units and the readings it didn't return), `get_snmp_device` (one device's readings over a range). Never the community or v3 keys |
+| Prometheus plugin | `list_prometheus_targets` (each target's state and its newest scrape: series kept, dropped and over the cap), `get_prometheus_target` (a target's stored series by metric, with labels and newest values). Never the token, and a password in a URL is masked |
 
 ### Operator: run playbooks, change monitoring