M3 Coordinator, API for reading/writing metrics and management

The M3 Coordinator implements the Prometheus Remote Read and Write HTTP endpoints, they also can be used however as general purpose metrics write and read APIs. Any metrics that are written to the remote write API can be queried using PromQL through the query APIs as well as being able to be read back by the Prometheus Remote Read endpoint. Remote Write Write a Prometheus Remote write query to M3.