summarize should include average frequency per metadata category value

[gregcaporaso]

Improvement Description
A nice addition to this viz would be a summary of the median frequency/sample count for each categorical metadata values (e.g., gut median frequency/sample (n=31): 5500; tongue median frequency/sample (n=22): 5600; left palm median frequency/sample (n=5): 4000; right palm median frequency/sample (n=42): 4500). I’m working on responding to reviewers on a paper, and just had a request for this information.

Current Behavior
This information isn't provided.

Proposed Behavior
I'd see this fitting on the existing Sample Detail tab, and could be shown for whichever metadata column is currently selected.