[Feature request] Allow metrics-type-prefixes using multiple flags

[snebel29]

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.

[SuperQ]

This mostly sounds like a configuration management problem. But I don't see an issue making --monitoring.metrics-type-prefixes be repeatable. There's no need to make separate flags for this.

I might suggest dropping the word type from the flag, as it's a bit "meta". --monitoring.metrics-prefixes makes more sense to me.