* [management] Add metrics for peer status updates and ephemeral cleanup
The session-fenced MarkPeerConnected / MarkPeerDisconnected path and
the ephemeral peer cleanup loop both run silently today: when fencing
rejects a stale stream, when a cleanup tick deletes peers, or when a
batch delete fails, we have no operational signal beyond log lines.
Add OpenTelemetry counters and a histogram so the same SLO-style
dashboards that already exist for the network-map controller can cover
peer connect/disconnect and ephemeral cleanup too.
All new attributes are bounded enums: operation in {connect,disconnect}
and outcome in {applied,stale,error,peer_not_found}. No account, peer,
or user ID is ever written as a metric label — total cardinality is
fixed at compile time (8 counter series, 2 histogram series, 4 unlabeled
ephemeral series).
Metric methods are nil-receiver safe so test composition that doesn't
wire telemetry (the bulk of the existing tests) works unchanged. The
ephemeral manager exposes a SetMetrics setter rather than taking the
collector through its constructor, keeping the constructor signature
stable across all test call sites.
* [management] Add OpenTelemetry metrics for ephemeral peer cleanup
Introduce counters for tracking ephemeral peer cleanup, including peers pending deletion, cleanup runs, successful deletions, and failed batches. Metrics are nil-receiver safe to ensure compatibility with test setups without telemetry.
