Measure GRPC server metrics at the HTTP level and client metrics at the service level

[rdettai]

Description

Currently, we have metrics called grpc_xxx that actually take measurement at the service level:

• this makes it hard to measure bytes transferd
• it sometimes measures calls that actually don't use the network stack, which makes the "grpc" name confusing.

This PR attemps to steamline the metrics:

• grpc_x metrics are now recorded by an http level layer on GRPC servers only
• we introduce service_x metrics that are recorded in clients only but use the "kind" label to distinguish "local" vs "remote" invocations.

How was this PR tested?

TODO

[rdettai]

TODO:

• tests
• verify that the client layers are properly registered (never attached to server stacks)
• see if we can somehow improve GrpcMetricsLayer to also track streams
• see if we can somehow improve GrpcMetricsLayer to also track compression