Make PromDash deal with empty metric names.

The Prometheus server is being changed to drop metric names after
transformations like sum(), avg(), etc., since the result of these
transformations is not the original metric anymore. Thus, the metric
name of timeseries may be empty now. PromDash needs to deal with this.
Merge pull request #214 from prometheus/julius/empty-metric-name

Make PromDash deal with empty metric names.