Additional labels

[eccles]

Just a request: would it be possible to add functionality that enables custom labels on the collected metrics?

I would like to classify requests by the customer id which is available in the header and/or context metadata so woud like to specify a function to get the value .

This is a newbie question as just trying to understand prom metrics collection...

I would be quite happy to submit a PR _ a modification to metrics,Config should do the trick I think.