interval: make the FormatDuration function public

A useful function that was ported from kbn.ts and can be used
to convert milliseconds into a kbn unit

pkg/tsdb/interval.go

@@ -59,11 +59,11 @@ func (ic *intervalCalculator) Calculate(timerange *TimeRange, minInterval time.D
 	interval := time.Duration((to - from) / defaultRes)
 
 	if interval < minInterval {
-		return Interval{Text: formatDuration(minInterval), Value: minInterval}
+		return Interval{Text: FormatDuration(minInterval), Value: minInterval}
 	}
 
 	rounded := roundInterval(interval)
-	return Interval{Text: formatDuration(rounded), Value: rounded}
+	return Interval{Text: FormatDuration(rounded), Value: rounded}
 }
 
 func GetIntervalFrom(dsInfo *models.DataSource, queryModel *simplejson.Json, defaultInterval time.Duration) (time.Duration, error) {
@@ -89,7 +89,8 @@ func GetIntervalFrom(dsInfo *models.DataSource, queryModel *simplejson.Json, def
 	return parsedInterval, nil
 }
 
-func formatDuration(inter time.Duration) string {
+// FormatDuration converts a duration into the kbn format e.g. 1m 2h or 3d
+func FormatDuration(inter time.Duration) string {
 	if inter >= year {
 		return fmt.Sprintf("%dy", inter/year)
 	}