add swarm-metrics, an agent CLI for the metrics store

swarm-metrics reads the swarm's VictoriaMetrics from inside an agent
container: query, query-range, series, labels and label-values, each one
GET against the store's Prometheus read API through the gateway, printing
the store's JSON. It authenticates the way swarm-logs does, with the
agent's own queue secret read from bao under the agent's leaf, sent as
a swarm-agent bearer.

On the metrics store's host the read API's auth map gains an agent arm
ahead of the Grafana one. It routes to swarm-metrics-agent-auth, a
swarm-logs-auth instance logged in under the existing swarm-metrics-auth
leaf, whose bao grant now includes swarm/agents/+/queue read. The check is
rate-limited per address like the log store's
(deploy.victoriametrics.agentAuth{Rate,Burst,VerdictTtl}).

nix/agent-modules/metrics.nix installs the wrapped binary when the hive
forwards HIVE_AGENT_METRICS_URL (from swarm.victoriametrics.domain) and a
store address, the same gate as swarm-logs. Adds the base:swarm-metrics
skill and the CLI reference with its freshness check.

Fixes #5395

nix/host-modules/glue-bao-tls.nix

@@ -285,8 +285,8 @@ in
         [ -s ${pkiDir}/logs-auth.pem ] || ${signLeaf} ${pkiDir} logs-auth \
           ${lib.escapeShellArg deployCfg.bao.logsAuthCommonName} "" clientAuth
 
-        # The metrics store's Grafana auth responder, which reads Grafana's
-        # metrics bearer. Minted here for the matrix-ctl leaf's reason: the
+        # The metrics store's auth responders, which read Grafana's metrics
+        # bearer and agents' queue secrets. Minted here for the matrix-ctl leaf's reason: the
         # metrics store is a swarm singleton, so elsewhere this is the file an
         # operator copies.
         [ -s ${pkiDir}/metrics-auth.pem ] || ${signLeaf} ${pkiDir} metrics-auth \