Improvements for $timeSeries template; sub-second data selection for DATETIME64

[tdamsma]

The default $timeSeries template rounds data to whole seconds, this seems entirely unnecessary. For a interval width of 50ms, the $timeSeries template resolves to:

(intDiv(toFloat64(time) * 1000, (1 * 1000)) * (1 * 1000))

So instead of using this $timeSeries template, I used the following:

(intDiv(toFloat64(time) * 1000, ${__interval_ms}) * ${__interval_ms})

This is translated to the following query:

intDiv(toFloat64(time) * 1000, 50) * 50

Looking at the code this is because the $interval variable is rounded to the nearest second. Perhaps a $interval_ms can be added as variable analog to how grafana both $__interval and $__interval_ms exist in grafana. And looking at the source, why is the grafana $__interval_ms not used as basis to compute these values?