Support more aggregation types for per bucket monitor sample documents

[AWSHurneyt]

The frontend visual editor translates the bucket monitor creation form selections into a composite aggregation (source); however, different aggregation types could potentially be configured using the extraction query editor.

The enhancement to return sample documents in per bucket monitor notification messages should be further enhanced to support all aggregation types.

Originally posted by @AWSHurneyt in #1450 (comment)