Add ability to select metric sources to API

[JonHass]

Need ability to query what metrics sources are available and subsequently request metric data filtered also by which sources the client wants.

[Artlands]

Need ability to query what metrics sources are available and subsequently request metric data filtered also by which sources the client wants.

Thanks for your reminder. We've optimized the filter function in the MetricsBuilder and merged it into the master branch.

[githubyongchen]

Jie - please keep the issue open till Mr. Hass has confirmed the issue resolved, and to be closed by him. Same guideline for other requests. Thx,