Add simple function to retrieve a labels_array

Retrieves a labels_array from a series_id.

docs/sql_schema.md

@@ -26,6 +26,7 @@ simply `cpu_usage` view). The view contains a the following column:
 
 A label value can be retrieved from the label name id using the `val`
 function. A full json for the series can be retrieved with `jsonb(labels)`.
+The array of labels can be retrieved from a series_id with `labels(series_id)`.
 
 For example:
 ```

pkg/pgmodel/end_to_end_tests/functions_test.go

@@ -244,7 +244,16 @@ func TestSQLJsonLabelArray(t *testing.T) {
 
 					if labelSet.Fingerprint() != fingerprintRes {
 						t.Fatalf("Json not equal: id %v\n got\n%v\nexpected\n%v", seriesID, string(jsonRes), string(jsonOrig))
+					}
+
+					err = db.QueryRow(context.Background(), "SELECT jsonb(labels($1))", seriesID).Scan(&jsonRes)
+					if err != nil {
+						t.Fatal(err)
+					}
+					fingerprintRes = getFingerprintFromJSON(t, jsonRes)
 
+					if labelSet.Fingerprint() != fingerprintRes {
+						t.Fatalf("Json not equal: id %v\n got\n%v\nexpected\n%v", seriesID, string(jsonRes), string(jsonOrig))
 					}
 
 					err = db.QueryRow(context.Background(), "SELECT (key_value_array(labels)).* FROM _prom_catalog.series WHERE id=$1",

pkg/pgmodel/migrations/sql/idempotent/base.sql

@@ -640,6 +640,20 @@ COMMENT ON FUNCTION SCHEMA_PROM.jsonb(labels SCHEMA_PROM.label_array)
 IS 'converts a labels array to a JSONB object';
 GRANT EXECUTE ON FUNCTION SCHEMA_PROM.jsonb(SCHEMA_PROM.label_array) TO prom_reader;
 
+--Returns the label_array given a series_id
+CREATE OR REPLACE FUNCTION SCHEMA_PROM.labels(series_id BIGINT)
+RETURNS SCHEMA_PROM.label_array AS $$
+    SELECT
+        labels
+    FROM
+        SCHEMA_CATALOG.series
+    WHERE id = series_id
+$$
+LANGUAGE SQL STABLE PARALLEL SAFE;
+COMMENT ON FUNCTION SCHEMA_PROM.labels(series_id BIGINT)
+IS 'fetches labels array for the given series id';
+GRANT EXECUTE ON FUNCTION SCHEMA_PROM.labels(series_id BIGINT) TO prom_reader;
+
 --Do not call before checking that the series does not yet exist
 CREATE OR REPLACE FUNCTION SCHEMA_CATALOG.create_series(
         metric_id int,