Allows Custom namespace for server/client metrics

[juanpedromoreno]

This PR allows providing externally the namespace for the registered metrics. Example:

val configList: List[ManagedChannelConfig] =
        AddInterceptor(
          MonitoringClientInterceptor(
            Configuration
              .defaultAllMetrics
              .withCollectorRegistry(CR)
              .withNamespace("custom_namespace")
          )
        )

.withNamespace("custom_namespace") is the combinator added here. If it's not provided, by default will be grpc, the same used until now.

[L-Lavigne]

Good stuff! 👍

[AdrianRaFo]

@juanpedromoreno it would be possible get the project name and add it to the default namespace?

[juanpedromoreno]

Frees-rpc users could do it that way.