feat(server): unify routing metrics and clarify replica capacity

Combine local relay setup and outbound peer requests in one counter, labeled by operation, route, target, outcome, and gRPC status. Preserve peer latency metrics.

Rename the rejection reason from global_capacity to replica_capacity to reflect the per-replica relay budget. Keep capacity limits unchanged.

Update tests and documentation.

Signed-off-by: divesh <dgude@nvidia.com>

Author: FrostGod