Azure Service Management REST API Get Metrics not retrieving results

Hi,

I have been trying to pull metric details for vm's using Azure Service Management REST API as given below

url = "https://management.core.windows.net/************/services/monitoring/metricvalues/query?resourceId=/hostedservices/*****/deployments/ ******/roles/******* &namespace=&names=['Disk Read Bytes']&timeGrain=PT5M&startTime="2015-09-07T06:49:30.908256Z"&endTime='2015-09-08T06:47:41.014201Z'"

But the response am getting is InvalidRequestCould not retrieve metrics

What am i doing wrong here. But am getting result for get metrics definition call correctly. So there will not be any issues with authentication or other parameters. Couldn't find any documentation in this regard also . Kindly help