The flag --monitoring.metrics-type-prefixes can easily become a long string hard to read, in some file formats like when provided in a k8s yaml manifests within a list, indented, etc.. can be tricky to split the string without adding extra spaces, making readability hard.

I think it'd be nice to being able to provide prefixes repeating a flag, may be create an additional --monitoring.metrics-type-prefix flag (singular) that can be repeated and which values would be merged/combined with env var STACKDRIVER_EXPORTER_MONITORING_METRICS_TYPE_PREFIXES and prefixes flag (plural).

I'm happy to PR if that makes sense, thanks.