Added comment about the source of the heap implementation.

sample.go

@@ -535,6 +535,7 @@ func newExpDecaySampleHeap(reservoirSize int) *expDecaySampleHeap {
 }
 
 // expDecaySampleHeap is a min-heap of expDecaySamples.
+// The internal implementation is copied from the standard library's container/heap
 type expDecaySampleHeap struct {
 	s []expDecaySample
 }