Allow filtering to decide which metrics should be exported

[xueweiz]

Is your feature request related to a problem? Please describe.

We are using OpenCensus Stackdriver Exporter on a open-source Kubernetes project node-problem-detector.
One problem we are facing at kubernetes/node-problem-detector#346 is that we (COS, GKE) only want to export a subset of OpenCensus views to Stackdriver Monitoring. However, the rest of the views are still useful for the open source community.

So we'd prefer to allow filtering on an exporter level, so that some metrics can be exported via Prometheus exporter, but skipped by Stackdriver exporter.

We cannot use OpenCensus agent because node-problem-detector has a high reliability requirement, and needs to be "the last thing that fails". Adding additional agent on its critical data path is against this requirement.

Describe the solution you'd like

We can add another option at here, taking a filter function to decide to skip a view or keep it.

Describe alternatives you've considered

I've considered to use OpenCensus agent, but cannot due to reliability requirement.

I've considered to change the behavior of GetMetricType option, and skip the exporting when it returns "". However it seems a little hacky.