Add per worker custom labels

[TLischkaRemerge]

This adds the ability for sidekiq worker classes to have a self.custom_labels field which lets them set custom labels to be attached to metrics they export. This makes it easier to alerts based on the metrics for specific workers or groups of workers

[SamSaffron]

I am good to merge this, but there is a merge conflict and also looks like we need a README update

[TLischkaRemerge]

Thanks for the review, I fixed the merge conflict and added a note to the README with an example of how to add the custom labels to a Sidekiq job

[SamSaffron]

thanks heaps @TLischkaRemerge !