[coordinator] Add support for Graphite Grafana plugin /find POST requests

[tejasbhosale009]

Added code that will support to grafana to show graphs using m3db ..

[claassistantio]

All committers have signed the CLA.

[fingon]

Grafana works fine without this too (using Prometheus), I guess this is needed for Graphite? What is the difference anyway, POSTed form field "query" as opposed to "query" parameter in the url?

[robskillington]

@tejasbhosale009 thanks for the contribution, this makes sense for Graphite find (I believe it's POST request?). I think we need a unit test before we can merge this for regression testing purposes, would you mind adding a test that sends a POST request?

Relevant datasource POST request for graphite find:
https://github.com/grafana/grafana/blob/master/public/app/plugins/datasource/graphite/datasource.ts#L324-L334

    const httpOptions: any = {
      method: 'POST',
      url: '/metrics/find',
      params: {},
      data: `query=${interpolatedQuery}`,
      headers: {
        'Content-Type': 'application/x-www-form-urlencoded',
      },
      // for cancellations
      requestId: options.requestId,
    };

[robskillington]

I added the test and merged master in.