Support additional grpc specific metrics #233
Description
Note: requires design #109.
The following grpc metrics should be contributed to the Zilla engine's awareness of available metrics via the telemetry-grpc component, consistent with other pluggable aspects of the runtime.
- grpc.duration: histogram
- grpc.request_size: histogram
- grpc.response_size: histogram
- grpc.active_requests: updown
- grpc.requests_per_rpc: histogram
- grpc.responses_per_rpc: histogram
Note: requests_per_rpc and responses_per_rpc measures the number of messages received per RPC. Should be 1 for all non-streaming RPCs