worker: add metrics for QUIC transport rotation outcomes

Description

Add an operational metric for worker QUIC transport rotation attempts. PR #1383 adds self-healing transport rotation, but successful recovery and replacement-socket bind failures are currently visible only in logs.

The metric should let operators distinguish a recovered stale network path from a worker that cannot create a replacement socket. A sustained successful rotation rate can also indicate an underlying network or load-balancer problem.

Definition of Done

• Add a counter for QUIC transport rotation attempts with a bounded result label such as success and bind_error.
• Define success as a replacement UDP socket being bound and installed for new dials.
• Pre-initialize every supported result value so the series is present before the first event.
• Do not use hostnames, addresses, function IDs, or request IDs as metric labels.
• Add focused tests for successful and failed rotation accounting.
• Document the intended operational use: successful rotations are diagnostic, while repeated bind failures require operator attention.

Resources (optional)

• #1383

Start with the worker QUIC transport rotation implementation introduced by PR #1383 and trace where replacement sockets are bound and installed. Add focused tests for successful and failed rotation accounting, pre-initialize the bounded result values, and document that successful rotations are diagnostic while repeated bind failures need operator attention.