[Metricbeat] Prometheus Remote Endpoint

[sorantis]

Today we can capture metrics from Prometheus servers, exporters, push gateways via our Prometheus module in Metricbeat. This requirement takes the next step by adding support for the remote write feature available in Prometheus.

Create a way to configure Elastic as a remote endpoint for Prometheus metrics in order to stream samples to Elasticsearch for long term storage and further analysis.

[exekias]

This could be a new metricset in the prometheus module, it would open an HTTP server listening for remote_write API calls. Similar to the http.server metricset. I did some tests around this in the past: https://github.com/elastic/beats/compare/master...exekias:prometheus-remote-storage?expand=1