Don't require project ID in StackdriverStatsExporter

[c24t]

The StackdriverStatsExporter shouldn't require a project ID if we can get the project from the environment with e.g. google.auth.default.

google.cloud.trace.client.Client gives us this for free, which is why we don't require a project ID in the (trace) StackdriverExporter. monitoring_v3.MetricServiceClient doesn't, which is presumably why we require one in StackdriverStatsExporter.

[vkryachko]

monitoring_v3.MetricServiceClient does not seem to require project_id or credentials and delegates to google.auth.default.

https://github.com/googleapis/google-cloud-python/blob/bea0d8c1d2b66e15d84ca3ab992f28138cf1b751/monitoring/google/cloud/monitoring_v3/gapic/metric_service_client.py#L112