gRPC metrics for in-flight requests to/from kas

Relates to #9 (closed).

Some additional gRPC metrics for kas. As you can see, with one agentk connected we see the grpc_server_requests_in_flight gauge with relevant labels set to 1. I've also instrumented the Gitaly client, which is less useful, but maybe can help troubleshooting. I decided to use the promgrpc library as it does not make sense to build the same thing. I hope once #32 (closed) is done, I can add these metrics upstream to that library and remove this code.

# HELP grpc_client_requests_in_flight TODO
# TYPE grpc_client_requests_in_flight gauge
grpc_client_requests_in_flight{grpc_client_user_agent="gitlab-kas/v0.0.0/deadbeef grpc-go/1.32.0",grpc_is_fail_fast="true",grpc_method="InfoRefsUploadPack",grpc_service="gitaly.SmartHTTPService"} 0
grpc_client_requests_in_flight{grpc_client_user_agent="gitlab-kas/v0.0.0/deadbeef grpc-go/1.32.0",grpc_is_fail_fast="true",grpc_method="TreeEntry",grpc_service="gitaly.CommitService"} 0
# HELP grpc_server_requests_in_flight TODO
# TYPE grpc_server_requests_in_flight gauge
grpc_server_requests_in_flight{grpc_method="GetConfiguration",grpc_service="agentrpc.Kas"} 1
grpc_server_requests_in_flight{grpc_method="GetObjectsToSynchronize",grpc_service="agentrpc.Kas"} 1