* Implement the ServerDiscovery.WatchServers gRPC endpoint
* Fix the ConnectCA.Sign gRPC endpoints metadata forwarding.
* Unify public gRPC endpoints around the public.TraceID function for request_id logging
Introduces a gRPC endpoint for signing Connect leaf certificates. It's also
the first of the public gRPC endpoints to perform leader-forwarding, so
establishes the pattern of forwarding over the multiplexed internal RPC port.