Provide a way to avoid a burst of calls to CreateMetricDescriptor from Stackdriver stats exporter

[sebright]

When an application has many processes that all use the OpenCensus Stackdriver stats exporter, it can create a burst of calls to Stackdriver's CreateMetricDescriptor RPC (https://cloud.google.com/monitoring/api/ref_v3/rpc/google.monitoring.v3#metricservice), which can exceed the limit on requests per minute. It would be nice if there was a way to avoid the burst of requests. For example, StackdriverStatsConfiguration could have an option to skip the call to CreateMetricDescriptor, and users could set it when they know that the metric descriptors already exist.

[songy23]

Note in Go stackdriver exporter there's already such an option: https://github.com/census-ecosystem/opencensus-go-exporter-stackdriver/pull/217/files#diff-47aceb412c41a069374790bf4dc6de1dR245.