gRPC Services Metrics using Prometheus

[juanpedromoreno]

This PR adds some new optional artifacts that you might want to add to your build in order to monitor your gRPC services.

Concretely, adding:

libraryDependencies += "io.frees" %% "frees-rpc-prometheus-server" % "x.y.z"

Your RPC services will be able to intercept every RPC call on the server side and collect metrics that you could render in your dashboard, using Prometheus.

Adding services without intercepting calls:

lazy val grpcConfigs: List[GrpcConfig] = List(
  AddService(RPCService.bindService[ConcurrentMonad])
)

Intercepting calls with Prometheus:

import freestyle.rpc.interceptors.implicits._

lazy val cr: CollectorRegistry = new CollectorRegistry()
lazy val monitorInterceptor = MonitoringServerInterceptor(configuration.withCollectorRegistry(cr))

lazy val grpcConfigs: List[GrpcConfig] = List(
  AddService(RPCService.bindService[ConcurrentMonad].interceptWith(monitorInterceptor))
)

Next steps:

• Support Client Metrics with Prometheus
• Support http://metrics.dropwizard.io/4.0.0/

Inspired by:

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

[fedefernandez]

prometheus-server only depends on server for testing, right? Or the idea is to include prometheus-server dependency instead of server?

[juanpedromoreno]

Not only for tests, you need server to make it work with prometheus-server, that's why it should be added transitively. prometheus-server doesn't make sense without server.

[fedefernandez]

Is this call to reportStartMetrics() intentional? in that case, I think we should add a blank line between the call and the private val