Fix LabelValueStats in posting stats (#12342)

Problem:
LabelValueStats - This will provide a list of the label names and memory used in bytes.
It is calculated by adding the length of all values for a given label name.
But internally Prometheus stores the name and the value independently for each series.

Solution:
MemPostings struct maintains the values to seriesRef map which is used
to get the number of series which contains the label values.
Using that LabelValueStats is calculated as: seriesCnt * len(value
name)

Signed-off-by: Baskar Shanmugam <baskar.shanmugam.career@gmail.com>

tsdb/index/postings.go

@@ -184,8 +184,9 @@ func (p *MemPostings) Stats(label string) *PostingsStats {
 			if n == label {
 				metrics.push(Stat{Name: name, Count: uint64(len(values))})
 			}
-			labelValuePairs.push(Stat{Name: n + "=" + name, Count: uint64(len(values))})
+			seriesCnt := uint64(len(values))
-			size += uint64(len(name))
+			labelValuePairs.push(Stat{Name: n + "=" + name, Count: seriesCnt})
+			size += uint64(len(name)) * seriesCnt
 		}
 		labelValueLength.push(Stat{Name: n, Count: size})
 	}

tsdb/index/postings_test.go

@@ -916,6 +916,35 @@ func BenchmarkPostings_Stats(b *testing.B) {
 	}
 }
 
+func TestMemPostingsStats(t *testing.T) {
+	// create a new MemPostings
+	p := NewMemPostings()
+
+	// add some postings to the MemPostings
+	p.Add(1, labels.FromStrings("label", "value1"))
+	p.Add(1, labels.FromStrings("label", "value2"))
+	p.Add(1, labels.FromStrings("label", "value3"))
+	p.Add(2, labels.FromStrings("label", "value1"))
+
+	// call the Stats method to calculate the cardinality statistics
+	stats := p.Stats("label")
+
+	// assert that the expected statistics were calculated
+	require.Equal(t, uint64(2), stats.CardinalityMetricsStats[0].Count)
+	require.Equal(t, "value1", stats.CardinalityMetricsStats[0].Name)
+
+	require.Equal(t, uint64(3), stats.CardinalityLabelStats[0].Count)
+	require.Equal(t, "label", stats.CardinalityLabelStats[0].Name)
+
+	require.Equal(t, uint64(24), stats.LabelValueStats[0].Count)
+	require.Equal(t, "label", stats.LabelValueStats[0].Name)
+
+	require.Equal(t, uint64(2), stats.LabelValuePairsStats[0].Count)
+	require.Equal(t, "label=value1", stats.LabelValuePairsStats[0].Name)
+
+	require.Equal(t, 3, stats.NumLabelPairs)
+}
+
 func TestMemPostings_Delete(t *testing.T) {
 	p := NewMemPostings()
 	p.Add(1, labels.FromStrings("lbl1", "a"))