Add pie charts to promdash

See title. Plz comment with anything I've missed, or additional functionality you would like like.

requested in #114 

@tomcz @juliusv @grobie @ohookins

This returns `undefined{foo="bar",...}`. On one hand, it would be `e.Metric['__name__']`, but on the other hand, you also delete that key above, so the metric name is gone from `e.Metric`. It needs to be preserved somehow.