MESOS-5731 Add metric types to GetMetrics v1 api

[Tuan-Anh Hoang-Vu]

Hi all,

I've finished implementing and testing this issue
https://issues.apache.org/jira/browse/MESOS-5731 and would like to find a
shepherd for my patch.

Should I go ahead and submit the patch?

Following Vinod's suggestion, I've changed Response::GetMetrics in
master.proto and v1/master.proto from

  message GetMetrics {
    repeated Metric metrics = 1;
  }

to

  message GetMetrics {
    repeated Metric counters = 1;
    repeated Metric gauges = 2;
    repeated Metric timers = 3;
  }