Serialize Monitoring API calls

[mrkm4ntr]

Monitoring API calls of TimeSeries that have same hash should be serialized.

• Points of same time series must be sent in order I got following errors from Monitoring API.

  rpc error: code = InvalidArgument desc = One or more TimeSeries could not be written: Points must be written in order. 
  One or more of the points specified had an older end time than the most recent point.: timeSeries[0-199]

• Returning recoverableError or not is non-deterministic Returning first error in errors currently. This may drop some Points failed by recoverable errors. https://github.com/Stackdriver/stackdriver-prometheus-sidecar/blob/1361301230bcfc978864a8f4c718aba98bc07a3d/stackdriver/client.go#L245-L247