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.
  

  stackdriver-prometheus-sidecar/stackdriver/client.go

  Lines 245 to 247 in 1361301

  	if err, ok := <-errors; ok {
  	return err
  	}