[ddsketch] Add getter for DDSketch zeroCount

The zeroCount field of a DDSketch can currently be indirectly accessed
using:
s.GetCount() - s.GetPositiveValueStore().TotalCount() - s.GetNegativeValueStore().TotalCount()
but this is cumbersome to use (and causes additional computations).

This change makes the zeroCount directly accessible through the GetZeroCount() getter.

ddsketch/ddsketch.go

@@ -31,6 +31,7 @@ type quantileSketch interface {
 	RelativeAccuracy() float64
 	IsEmpty() bool
 	GetCount() float64
+	GetZeroCount() float64
 	GetSum() float64
 	GetPositiveValueStore() store.Store
 	GetNegativeValueStore() store.Store
@@ -202,6 +203,13 @@ func (s *DDSketch) GetCount() float64 {
 	return s.zeroCount + s.positiveValueStore.TotalCount() + s.negativeValueStore.TotalCount()
 }
 
+// GetZeroCount returns the number of zero values that have been added to this sketch.
+// Note: values that are very small (lower than MinIndexableValue if positive, or higher than -MinIndexableValue if negative)
+// are also mapped to the zero bucket.
+func (s *DDSketch) GetZeroCount() float64 {
+	return s.zeroCount
+}
+
 // Return true iff no value has been added to this sketch.
 func (s *DDSketch) IsEmpty() bool {
 	return s.zeroCount == 0 && s.positiveValueStore.IsEmpty() && s.negativeValueStore.IsEmpty()
@@ -538,6 +546,13 @@ func (s *DDSketchWithExactSummaryStatistics) GetCount() float64 {
 	return s.summaryStatistics.Count()
 }
 
+// GetZeroCount returns the number of zero values that have been added to this sketch.
+// Note: values that are very small (lower than MinIndexableValue if positive, or higher than -MinIndexableValue if negative)
+// are also mapped to the zero bucket.
+func (s *DDSketchWithExactSummaryStatistics) GetZeroCount() float64 {
+	return s.DDSketch.zeroCount
+}
+
 func (s *DDSketchWithExactSummaryStatistics) GetSum() float64 {
 	return s.summaryStatistics.Sum()
 }