rpc: report psrpc request delivery, expiry and claim outcomes

[u9g]

Draft — depends on livekit/psrpc#118. go.mod currently pins a pseudo-version of that branch so this compiles; it must be re-pinned to a tagged psrpc release before merge.

Why

Three server-side lifecycle events in psrpc are invisible to the interceptor chain, because in each case the handler is never invoked:

• a request read off the bus
• a request dropped for arriving past its expiry
• settlement of the claim handshake

The third is the one that matters. When a caller gets ErrNoResponse — no server bid within the selection window — nothing is recorded on either side. The request exists in no log, metric or trace, so "the request never arrived" and "a server bid too late" are indistinguishable, despite having different causes and different fixes.

Change

PSRPCMetricsObserver implements the new psrpc.RequestObserver. psrpc wires it automatically for any observer passed to middleware.WithServerMetrics, so every service already using WithServerObservability picks this up with no call-site change.

livekit_psrpc_requests_received_total{service,method}
livekit_psrpc_requests_expired_total{service,method}
livekit_psrpc_claim_total{service,method,outcome}         # granted | lost_to_peer | abandoned
livekit_psrpc_claim_wait_time_ms{service,method,outcome}

abandoned and pre-dispatch expiry also log at warn, alongside the existing server error / client error lines in rpc/logging.go.

Notes on the shape:

• Lifecycle series carry no role label — they are server-side by construction, and a constant label would be misleading.
• Histogram buckets are 1ms–3s: a granted claim settles in single-digit ms, an abandoned one runs to the caller's selection timeout (1s by default), so the mass sits at both ends.
• lost_to_peer is expected on broadcast RPCs; on a queue RPC it means more than one member received the same request.

Tests

TestRequestObserverMetrics gathers from the registry and asserts each series actually registers and increments — not merely that the methods compile. A silent regression here would be invisible in production, since these are the only signals for a request whose handler never runs.

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