Add full regex support for mapping configuration

[bakins]

This is a quick hack to allow the use of full regex expressions for the mapping configuration. Currently, I'm imploring some legacy metrics and this makes it a bit less laborious.

[brian-brazil]

Can you give examples of metrics you're seeing that aren't following the standard graphite dotted string convention?

[bakins]

We have some metrics that are currently a bit "messy." Something like: foo.bar.mysql_something.foo_bar and we actually want to map the label as msyql=something rather than mysql=mysql_something. We could do that in the prometheus scrape config, but that seemed more fragile.

This is admittedly a bit of a hack for collecting from a legacy system, but we found it helpful.

[brian-brazil]

If it's just one unusual system you have, it may be better for you to handle this with a metric_relabel_config than to complicate the graphite exporter and all other exporters using this code.

[bakins]

Thanks for taking the time to look at this and provide feedback. We will take a look at what you suggested.