Handle tags

Currently, the only way to send tags to Canopsis from prometheus is to use this conf

tags:
  type: copy
  value : labels

it will convert labels to the correct format for tags in Canopsis.

We would like now to be able to configure which tag to send to Canopsis.

Any attr from prometheus should be usable as tag.

For example: labels.label1, labels.labeln, annotations.title

Could you please suggest some changes to allow this in configuration ?