Noficiation rule tag matching should be drop downs

Currently we have the user manually type out the tags that should be matched in a rule. These fields should be populated with the tag keys and values available in the `statuses` measurement in the `_monitoring` bucket.


Start at the rule tag-matching fields and the `statuses` measurement in the `_monitoring` bucket. Trace how available tag keys and values are obtained, then verify that selecting them in dropdowns produces the same rule matches as manual entry.

Written by the indexing model from the issue text.