[prometheus-to-sd] containerNameLabel being checked in ValidateMetricDescriptors (in metric_descriptor_cache.go)

According to the README, prometheus labels are not included as labels in stackdriver ("Note that prometheus labels used for the monitored resource are not included as labels in stackdriver.").

The behaviour in ValidateMetricDescriptors (in metric_descriptor_cache.go) seems to clash with this, as it does check the containerNameLabel label and warns us with the message `Definition of the metric %s was changed and metric is not going to be pushed`