Expose resource tags in metrics labels

[demeringo]

Problem

Exposing resource tags was excluded from issue #405 (I did not have time to fix a formating issue that prevented metrics scrapping, so I delayed the feature)).

Solution

Expose tag and tag values as a label of the resource metrics.

Alternatives

Additional context or elements

[demeringo]

Will expose tags in a semi-column separated format like:

resource_tags="tag_key_1:tag_value_1;tag_key_2:tag_value_2;"