Prometheus plugin: expose Kubernetes annotations as tags

## Feature Request

It would be nice to have the option to also import and expose the Kubernetes annotations for discovered containers as tags.

### Proposal:
Expose annotations as tags

### Current behavior:
-
### Desired behavior:
Expose annotations as tags

### Use case:
This would enable us to tag containers with our Terraform deploys and filter the data in Elastic based on the annotations.

Start by locating the Prometheus plugin's Kubernetes container discovery and existing tag construction. Trace how discovered metadata is selected for tags, then define the option and behavior needed to expose annotations and verify that annotation-derived tags can filter the resulting data in Elastic.

Written by the indexing model from the issue text.