Allow templatize in labels

[bLd75]

Currently it's not possible to use variables in metrics labels, but some APIs return responses without the input.
It would be great to be able to use variable values in metric labels.

Example

      labels:
        input_value: '{{ .myVal | first }}'

Error returned

caller=collector.go:134 level=error msg="Failed to parse jsonpath" err="unrecognized character in action: U+007B '{'" path="{{ .myVal | first }}"

Thanks in advance!

[davordbetter]

This would be very useful.
Additionally, being able to templatize headers would be great too.