Expose NewChainSampleIterator func (#9475)

* expose NewChainSampleIterator func

Signed-off-by: Ben Ye <ben.ye@bytedance.com>

* add comment

Signed-off-by: Ben Ye <ben.ye@bytedance.com>

* update comments

Signed-off-by: Ben Ye <ben.ye@bytedance.com>

storage/merge.go

@@ -431,7 +431,7 @@ func ChainedSeriesMerge(series ...Series) Series {
 			for _, s := range series {
 				iterators = append(iterators, s.Iterator())
 			}
-			return newChainSampleIterator(iterators)
+			return NewChainSampleIterator(iterators)
 		},
 	}
 }
@@ -447,7 +447,10 @@ type chainSampleIterator struct {
 	lastt int64
 }
 
-func newChainSampleIterator(iterators []chunkenc.Iterator) chunkenc.Iterator {
+// NewChainSampleIterator returns a single iterator that iterates over the samples from the given iterators in a sorted
+// fashion. If samples overlap, one sample from overlapped ones is kept (randomly) and all others with the same
+// timestamp are dropped.
+func NewChainSampleIterator(iterators []chunkenc.Iterator) chunkenc.Iterator {
 	return &chainSampleIterator{
 		iterators: iterators,
 		h:         nil,

storage/merge_test.go

@@ -631,7 +631,7 @@ func TestChainSampleIterator(t *testing.T) {
 			expected: []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}, sample{3, 3}},
 		},
 	} {
-		merged := newChainSampleIterator(tc.input)
+		merged := NewChainSampleIterator(tc.input)
 		actual, err := ExpandSamples(merged, nil)
 		require.NoError(t, err)
 		require.Equal(t, tc.expected, actual)
@@ -677,7 +677,7 @@ func TestChainSampleIteratorSeek(t *testing.T) {
 			expected: []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}, sample{3, 3}},
 		},
 	} {
-		merged := newChainSampleIterator(tc.input)
+		merged := NewChainSampleIterator(tc.input)
 		actual := []tsdbutil.Sample{}
 		if merged.Seek(tc.seek) {
 			t, v := merged.At()