Replace comma-delimited string flags with repeatable flags

[tommyzli]

Closes #44
The original issue is pretty old but the feature request still holds up, I too find it more ergonomic to manage repeatable flags rather than a large comma-delimited string. As suggested in the issue, I also changed metrics-type-prefixes to just metrics-prefixes

[SuperQ]

I like it, mind doing a rebase?

[tommyzli]

@SuperQ Done, thanks

[SuperQ]

I wonder if it would be worth keeping the original flag as-is, and append the strings.Split() to the new flag results. Then mark the original flag as deprecated and print a warning in the log.

[kgeckhart]

Thanks!