Allow for custom dynamic labels on default metrics

[iocentos]

Hi there,

Thanks for this package, it works like a charm.

I have a use case where I'd like to be able to inject custom labels into existing default metrics.
We get an X-Request-Scope header as part of the requests which I'd like to pass as a label into the built in metrics. This would allow us to build dashboards and see metrics based on request scope.

Is this a possibility today or what kind of changes would it require to allow for such option?

Thank you

[martian711]

#287 this pull request works for you