Support matching on multiple labels

[matthewjstanford]

trafficstars

In our environment we have several different metric labels that could all indicate what tenant the metric should belong to. Could we add support for multiple label matches, in a sort of hierarchy?

For example, here is the current configuration:

tenant:
  label: namespace

But we would like to prefer a label of tenant if it exists, and also some of our metrics labels are rewritten exported_label, so we'd prefer that as well.

Looking for something like this:

tenant:
  label_list:
    - tenant
    - exported_namespace
    - namespace

This wouldn't need to be a breaking change. We can use the list logic only if the list is set, otherwise it will default to the original behavior.

Does this sound like a good addition? If so I can get a PR together for this.