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>

plugins/prometheus/__init__.py

@@ -68,3 +68,13 @@ def get_audit_entities() -> dict:
     from .models import PrometheusTarget, PrometheusWidgetTemplate
     return {"prometheus_target": (PrometheusTarget, "/plugins/prometheus/targets/{}"),
             "prometheus_template": (PrometheusWidgetTemplate, "/plugins/prometheus/templates/custom:{}")}
+
+
+def register_mcp_tools(register) -> None:
+    from .mcp_tools import register_tools
+    register_tools(register)
+
+
+async def mcp_overview(db) -> list[dict]:
+    from .mcp_tools import mcp_overview as findings
+    return await findings(db)