Allow group-by in the count mode of Histogram panel.

[athana]

Currently, the 'count' mode in Histogram panel cannot do group-by field, this is only allowed in the 'values' mode. For example, the use case here is to plot event counts and group-by id field over time.

[jayurbain]

It would be nice to do this for the terms panel as well, i.e., group-by.