Alerting: Add rule_matcher filter to Prometheus rules API (#115297)

**What is this feature?**

Add `rule_matcher` filter to the Prometheus-compatible list rules API: `/api/prometheus/grafana/api/v1/rules`. It allows to filter rules by static labels (not by alert instance labels).

**Special notes:**
  - Equality (`=`) and inequality (`!=`) matchers are pushed down to the database. Regex matchers (`=~`, `!~`) are applied in-memory at the API layer.
  - SQLite: Uses GLOB pattern matching
  - MySQL / PostgreSQL: Use JSON functions to compare label values


---------

Co-authored-by: Konrad Lalik <konradlalik@gmail.com>

pkg/tests/api/alerting/api_prometheus_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 	"sort"
 	"testing"
 	"time"
@@ -364,8 +365,6 @@ func TestIntegrationPrometheusRules(t *testing.T) {
 func TestIntegrationPrometheusRulesPagination(t *testing.T) {
 	testutil.SkipIntegrationTestInShortMode(t)
 
-	testinfra.SQLiteIntegrationTest(t)
-
 	dir, path := testinfra.CreateGrafDir(t, testinfra.GrafanaOpts{
 		DisableLegacyAlerting: true,
 		EnableUnifiedAlerting: true,
@@ -388,23 +387,30 @@ func TestIntegrationPrometheusRulesPagination(t *testing.T) {
 	require.NoError(t, err)
 
 	// Create 3 rule groups with different numbers of rules
-	// Group 1: 5 rules, Group 2: 3 rules, Group 3: 2 rules (total: 10 rules)
+	// Group 1: 5 rules with team=backend
+	// Group 2: 3 rules with team=frontend
+	// Group 3: 2 rules with team=platform
 	for groupIdx := 1; groupIdx <= 3; groupIdx++ {
 		var rulesCount int
+		var team string
 		switch groupIdx {
 		case 1:
 			rulesCount = 5
+			team = "backend"
 		case 2:
 			rulesCount = 3
+			team = "frontend"
 		case 3:
 			rulesCount = 2
+			team = "platform"
 		}
 
 		rules := make([]apimodels.PostableExtendedRuleNode, rulesCount)
 		for i := 0; i < rulesCount; i++ {
 			rules[i] = apimodels.PostableExtendedRuleNode{
 				ApiRuleNode: &apimodels.ApiRuleNode{
-					For: &interval,
+					For:    &interval,
+					Labels: map[string]string{"team": team},
 				},
 				GrafanaManagedAlert: &apimodels.PostableGrafanaRule{
 					Title:     fmt.Sprintf("rule-%d-%d", groupIdx, i+1),
@@ -514,6 +520,61 @@ func TestIntegrationPrometheusRulesPagination(t *testing.T) {
 		require.Equal(t, http.StatusOK, resp.StatusCode)
 		require.Len(t, result.Data.RuleGroups, 0, "should return no groups")
 	})
+
+	t.Run("with rule_matcher filter returns only matching rules", func(t *testing.T) {
+		matcher := url.QueryEscape(`{"name":"team","value":"frontend","isRegex":false,"isEqual":true}`)
+		promRulesURL := fmt.Sprintf("http://grafana:password@%s/api/prometheus/grafana/api/v1/rules?rule_matcher=%s", grafanaListedAddr, matcher)
+		// nolint:gosec
+		resp, err := http.Get(promRulesURL)
+		require.NoError(t, err)
+		t.Cleanup(func() {
+			err := resp.Body.Close()
+			require.NoError(t, err)
+		})
+
+		var result apimodels.RuleResponse
+		err = json.NewDecoder(resp.Body).Decode(&result)
+		require.NoError(t, err)
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+
+		// Should only return group-2 (team=frontend, 3 rules)
+		foundGroups := []string{}
+		total := 0
+		for _, group := range result.Data.RuleGroups {
+			foundGroups = append(foundGroups, group.Name)
+			total += len(group.Rules)
+		}
+		require.Equal(t, []string{"group-2"}, foundGroups)
+		require.Equal(t, 3, total)
+	})
+
+	t.Run("with rule_matcher regex filter", func(t *testing.T) {
+		// Filter with regex team=~plat.* (should match group-3 with team=platform)
+		matcher := url.QueryEscape(`{"name":"team","value":"plat.*","isRegex":true,"isEqual":true}`)
+		promRulesURL := fmt.Sprintf("http://grafana:password@%s/api/prometheus/grafana/api/v1/rules?rule_matcher=%s", grafanaListedAddr, matcher)
+		// nolint:gosec
+		resp, err := http.Get(promRulesURL)
+		require.NoError(t, err)
+		t.Cleanup(func() {
+			err := resp.Body.Close()
+			require.NoError(t, err)
+		})
+
+		var result apimodels.RuleResponse
+		err = json.NewDecoder(resp.Body).Decode(&result)
+		require.NoError(t, err)
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+
+		// Should only return group-3 (team=platform matches plat.*)
+		foundGroups := []string{}
+		total := 0
+		for _, group := range result.Data.RuleGroups {
+			foundGroups = append(foundGroups, group.Name)
+			total += len(group.Rules)
+		}
+		require.Equal(t, []string{"group-3"}, foundGroups)
+		require.Equal(t, 2, total)
+	})
 }
 
 func TestIntegrationPrometheusRulesFilterByDashboard(t *testing.T) {