Handle empty matcher

Signed-off-by: Saswata Mukherjee <saswataminsta@yahoo.com>

Author: saswatamcode

pkg/prometheus/loader.go

@@ -55,9 +55,15 @@ func (p *RealLoader) WithGuardrails(g *Guardrails) *RealLoader {
 }
 
 func (p *RealLoader) ListMetrics(ctx context.Context, nameRegex string) ([]string, error) {
-	// Create a matcher for the __name__ label with the provided regex
-	matcher := fmt.Sprintf("{__name__=~\"%q\"}", nameRegex)
-	labelValues, _, err := p.client.LabelValues(ctx, "__name__", []string{matcher}, time.Now().Add(-ListMetricsTimeRange), time.Now())
+	var matches []string
+
+	// For blanket regex patterns like ".*", use empty matcher to get all metrics to not get 4xx.
+	if nameRegex != ".*" && nameRegex != ".+" && nameRegex != "" {
+		matcher := fmt.Sprintf("{__name__=~\"%q\"}", nameRegex)
+		matches = []string{matcher}
+	}
+
+	labelValues, _, err := p.client.LabelValues(ctx, "__name__", matches, time.Now().Add(-ListMetricsTimeRange), time.Now())
 	if err != nil {
 		return nil, fmt.Errorf("error fetching metric names: %w", err)
 	}

tests/e2e/e2e_test.go

@@ -66,28 +66,33 @@ func TestHealthEndpoint(t *testing.T) {
 	}
 }
 
-func TestListMetrics(t *testing.T) {
-	resp, err := mcpClient.CallTool(t, 1, "list_metrics", map[string]any{
+func TestListMetricsReturnsKnownMetrics(t *testing.T) {
+	resp, err := mcpClient.CallTool(t, 2, "list_metrics", map[string]any{
 		"name_regex": ".*",
 	})
 	if err != nil {
 		t.Fatalf("Failed to call list_metrics: %v", err)
 	}
 
 	if resp.Error != nil {
-		t.Errorf("MCP error: %s", resp.Error.Message)
+		t.Fatalf("MCP error: %s", resp.Error.Message)
 	}
 
-	if resp.Result == nil {
-		t.Error("Expected result, got nil")
-	}
+	// Verify known metrics from kube-prometheus are present
+	resultJSON, _ := json.Marshal(resp.Result)
+	resultStr := string(resultJSON)
 
-	t.Logf("list_metrics returned successfully")
+	expectedMetrics := []string{"up", "prometheus_build_info"}
+	for _, metric := range expectedMetrics {
+		if !strings.Contains(resultStr, metric) {
+			t.Errorf("Expected metric %q not found in results", metric)
+		}
+	}
 }
 
-func TestListMetricsReturnsKnownMetrics(t *testing.T) {
+func TestListMetricsReturnsKnownMetricsWithMatcher(t *testing.T) {
 	resp, err := mcpClient.CallTool(t, 2, "list_metrics", map[string]any{
-		"name_regex": ".*",
+		"name_regex": "prometheus.*",
 	})
 	if err != nil {
 		t.Fatalf("Failed to call list_metrics: %v", err)
@@ -101,7 +106,7 @@ func TestListMetricsReturnsKnownMetrics(t *testing.T) {
 	resultJSON, _ := json.Marshal(resp.Result)
 	resultStr := string(resultJSON)
 
-	expectedMetrics := []string{"up", "prometheus_build_info"}
+	expectedMetrics := []string{"prometheus_build_info"}
 	for _, metric := range expectedMetrics {
 		if !strings.Contains(resultStr, metric) {
 			t.Errorf("Expected metric %q not found in results", metric)