Simplify use of histogramQuantile

[nikolay-t]

Currently use of histogramQuantile is not simple. It is not directly compatible with typical use case for values of tag, so we have to map for both countColumn and upperBoundColumn.

This is because normally you would put values of "le" column in InfluxDB (directly or via Telegraf) as tag=value, and value of a tag is always "string". It is not space-efficient to put it as field value. Also "count" column is normally "long", not "float" as required by histogramQuantile. So in both case, we have to apply map(), which is not convenient.

Please consider supporting "string" values for upperBoundColumn and "long" for countColumn.