Select metrics from specific subsystems

[stephen-soltesz]

Today, stackdriver_exporter allows specifying --monitoring.metrics-type-prefixes. And, some prefixes include multiple subsystems. For example, logging.googleapis.com/log_entry_count exports metrics for the following subsystems among others: gae_app, cloud_function, gce_instance, gke_container, k_8_s_cluster.

It would be a helpful feature to optionally allow selecting a subset of subsystems.

One way could be to specify metrics-type-prefixes with the subsystem. For example, logging.googleapis.com/log_entry_count@cloud_function. If the subsystem is unspecified then it implies "all subsystems".