add grpc metrics

[drewwells]

This middleware should be added early in the flow for grpc interceptors to ensure errors during authn/authz are captured.

https://github.com/grpc-ecosystem/go-grpc-prometheus

[kd7lxl]

Where exactly would this project need to include the prometheus gRPC interceptor? This is already part of the sister-project's code generator: infobloxopen/atlas-cli#53