Support ordering by tag columns in totals queries

[evanh]

For widgets we would need to support order by the grouped-by fields and aggregate value.

Specifically this is for ordering a totals query that also has a group by. E.g.

SELECT project, tags[something], sum(value) AS total
GROUP BY project, tags[something]
ORDER BY project, total

This is really a symptom of the frontend wanting to display tables of metrics totals, ordered by arbitrary columns. The better solution might be to solve the larger problem.