[Cardinality API] Include missing labels in label values

CHANGELOG.md

@@ -70,6 +70,7 @@
 * [ENHANCEMENT] Query-frontend: added `remote_read` to `op` supported label values for the `cortex_query_frontend_queries_total` metric. #8412
 * [ENHANCEMENT] Query-frontend: log the overall length and start, end time offset from current time for remote read requests. The start and end times are calculated as the miminum and maximum times of the individual queries in the remote read request. #8404
 * [ENHANCEMENT] Storage Provider: Added option `-<prefix>.s3.dualstack-enabled` that allows disabling S3 client from resolving AWS S3 endpoint into dual-stack IPv4/IPv6 endpoint. Defaults to true. #8405
+* [ENHANCEMENT] Cardinality API: Include series counts for series without a value in label_values endpoint #8450
 * [BUGFIX] Distributor: prometheus retry on 5xx and 429 errors, while otlp collector only retry on 429, 502, 503 and 504, mapping other 5xx errors to the retryable ones in otlp endpoint. #8324 #8339
 * [BUGFIX] Distributor: make OTLP endpoint return marshalled proto bytes as response body for 4xx/5xx errors. #8227
 * [BUGFIX] Rules: improve error handling when querier is local to the ruler. #7567

pkg/ingester/ingester_test.go

@@ -3842,6 +3842,7 @@ func TestIngester_LabelValuesCardinality(t *testing.T) {
 						"200": 1,
 						"300": 1,
 						"500": 1,
+						"":    1,
 					},
 				},
 				{
@@ -3855,6 +3856,7 @@ func TestIngester_LabelValuesCardinality(t *testing.T) {
 					LabelName: "env",
 					LabelValueSeries: map[string]uint64{
 						"prod": 2,
+						"":     2,
 					},
 				},
 			},
@@ -3867,7 +3869,7 @@ func TestIngester_LabelValuesCardinality(t *testing.T) {
 			expectedItems: []*client.LabelValueSeriesCount{
 				{
 					LabelName:        "status",
-					LabelValueSeries: map[string]uint64{"300": 1},
+					LabelValueSeries: map[string]uint64{"300": 1, "": 1},
 				},
 			},
 		},

pkg/ingester/label_names_and_values.go

@@ -136,6 +136,25 @@ func labelValuesCardinality(
 		if err != nil {
 			return err
 		}
+
+		// Check if empty value for label is allowed according to matchers. This
+		// is an optimization only to avoid fetching postings, since the posting
+		// list would be empty anyway.
+		emptyValueAllowedByMatchers := true
+		for _, matcher := range matchers {
+			if matcher.Name != lblName {
+				continue
+			}
+			if !matcher.Matches("") {
+				emptyValueAllowedByMatchers = false
+				break
+			}
+		}
+
+		if emptyValueAllowedByMatchers {
+			lblValues = append(lblValues, "")
+		}
+
 		// For each value count total number of series storing the result into cardinality response item.
 		var respItem *client.LabelValueSeriesCount
 

pkg/ingester/label_names_and_values_test.go

@@ -166,11 +166,11 @@ func TestIngester_LabelValuesCardinality_SentInBatches(t *testing.T) {
 	require.Equalf(t, maxBatchLabelValues, len(mockServer.SentResponses[0].Items[0].LabelValueSeries), "First label of first response should contain max amount of labels %d", maxBatchLabelValues)
 
 	require.Equal(t, 2, len(mockServer.SentResponses[1].Items), "Second server response should contain two label names")
-	require.Equal(t, 1, len(mockServer.SentResponses[1].Items[0].LabelValueSeries), "First label of second response should contain one value")
+	require.Equal(t, 2, len(mockServer.SentResponses[1].Items[0].LabelValueSeries), "First label of second response should contain two values")
 	require.Equalf(t, maxBatchLabelValues-1, len(mockServer.SentResponses[1].Items[1].LabelValueSeries), "Second label of second response should contain %d values", maxBatchLabelValues-1)
 
 	require.Equal(t, 1, len(mockServer.SentResponses[2].Items), "Third response should contain only one label")
-	require.Equal(t, 1, len(mockServer.SentResponses[2].Items[0].LabelValueSeries), "First label of third response should contain one label")
+	require.Equal(t, 2, len(mockServer.SentResponses[2].Items[0].LabelValueSeries), "First label of third response should contain two values")
 }
 
 func TestIngester_LabelValuesCardinality_AllValuesToBeReturnedInSingleMessage(t *testing.T) {
@@ -194,6 +194,8 @@ func TestIngester_LabelValuesCardinality_AllValuesToBeReturnedInSingleMessage(t
 			},
 			expectedItems: []*client.LabelValueSeriesCount{
 				{LabelName: "label-a", LabelValueSeries: map[string]uint64{"a-0": 1}},
+				// label-b is not present, but it was requested, so it should be returned with an empty value.
+				{LabelName: "label-b", LabelValueSeries: map[string]uint64{"": 1}},
 			},
 		},
 		"all values returned in a single message if response size is less then batch size": {
@@ -206,11 +208,11 @@ func TestIngester_LabelValuesCardinality_AllValuesToBeReturnedInSingleMessage(t
 			expectedItems: []*client.LabelValueSeriesCount{
 				{
 					LabelName:        "label-a",
-					LabelValueSeries: map[string]uint64{"a-0": 1, "a-1": 2, "a-2": 3},
+					LabelValueSeries: map[string]uint64{"a-0": 1, "a-1": 2, "a-2": 3, "": 3},
 				},
 				{
 					LabelName:        "label-b",
-					LabelValueSeries: map[string]uint64{"b-0": 1, "b-1": 2},
+					LabelValueSeries: map[string]uint64{"b-0": 1, "b-1": 2, "": 6},
 				},
 			},
 		},
@@ -226,6 +228,10 @@ func TestIngester_LabelValuesCardinality_AllValuesToBeReturnedInSingleMessage(t
 					LabelName:        "label-a",
 					LabelValueSeries: map[string]uint64{"a-1": 2, "a-2": 3},
 				},
+				{
+					LabelName:        "label-b",
+					LabelValueSeries: map[string]uint64{"": 5},
+				},
 			},
 		},
 	}