Allow filtering of different stats via HTTP requests

By default, "average" and "numPoints" are the stats that would be sent out as responses. 

If user specifies a select query parameter like

GET /v1.0/:tenantId/experimental/views/metric_data/:metricName?from= 1378235503000&to= 1378335503000&points=100&select=average&select=variance

we'd send out both average and variance as part of JSON response. 

Note that numPoints is also a 'selectable' stat. 'timestamp' key is always returned. 

You can also specify stats like select=average,min,max but such a notation is not recommended.