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

Ah, `cpu_idle` is a special case, probably because the node exporters are outdated. The `name` label is usually not there (try metrics from another job). So yeah, `__name__` is the field to use to get to the metric name.