feature request: extra labels option

[danielmotaleite]

Hi

I'm running in AWS and i would like that the exporter could take some command line parameters with "extra label" to inject in the metrics.
The idea is to inject the availability-zone and possibly the region in the metrics directly. So i would curl the aws metadata and get the info and pass it in the exporter inside the container.

Something like -label "az=$az, region=$region" would be perfect. This would make easier to find where the requests are coming from

Thanks for the exporter!