histogram: enable simple value-based histograms

This change enhances the histogram panel to support value-based
histograms in addition to time-series histograms.  This means that
instead of bucketing the field across time intervals, simply bucket all
values together into a single set of buckets based on the value itself,
while still honoring the overall dashboard time-range criteria.  This
type of graph provides a high-level perspective of a field's value.