Grafana Mimir: Graphite, Datadog, Influx, and Prometheus Metrics in One Place

Full title was a bit wordy and did not fit the limit, I've attempted to keep the essence of it.

This is the launch of OSS write proxies that allows data from other agents to be ingested into the Grafana Mimir ( https://github.com/grafana/mimir ) metrics TSDB.