New documentation on surfacers and stackdriver

[manugarg]

Hi folks,

I recently added some documentation on surfacers (mechanism to export metrics to other monitoring systems) and Stackdriver surfacer:

Surfacers:
https://cloudprober.org/surfacers/overview/

Stackdriver (Google Cloud Monitoring) Surfacer:
https://cloudprober.org/surfacers/stackdriver/