Refactor processExternalLabels method with slice copy for left labels

Signed-off-by: songjiayang <songjiayang1@gmail.com>

storage/remote/queue_manager.go

@@ -727,14 +727,8 @@ func processExternalLabels(ls labels.Labels, externalLabels labels.Labels) label
 			j++
 		}
 	}
-	for ; i < len(ls); i++ {
-		result = append(result, labels.Label{
-			Name:  ls[i].Name,
-			Value: ls[i].Value,
-		})
-	}
-	result = append(result, externalLabels[j:]...)
-	return result
+
+	return append(append(result, ls[i:]...), externalLabels[j:]...)
 }
 
 func (t *QueueManager) updateShardsLoop() {

storage/remote/queue_manager_test.go

@@ -794,6 +794,34 @@ func TestProcessExternalLabels(t *testing.T) {
 			externalLabels: labels.Labels{{Name: "a", Value: "c"}},
 			expected:       labels.Labels{{Name: "a", Value: "b"}},
 		},
+
+		// Test empty externalLabels.
+		{
+			labels:         labels.Labels{{Name: "a", Value: "b"}},
+			externalLabels: labels.Labels{},
+			expected:       labels.Labels{{Name: "a", Value: "b"}},
+		},
+
+		// Test empty labels.
+		{
+			labels:         labels.Labels{},
+			externalLabels: labels.Labels{{Name: "a", Value: "b"}},
+			expected:       labels.Labels{{Name: "a", Value: "b"}},
+		},
+
+		// Test labels is longer than externalLabels.
+		{
+			labels:         labels.Labels{{Name: "a", Value: "b"}, {Name: "c", Value: "d"}},
+			externalLabels: labels.Labels{{Name: "e", Value: "f"}},
+			expected:       labels.Labels{{Name: "a", Value: "b"}, {Name: "c", Value: "d"}, {Name: "e", Value: "f"}},
+		},
+
+		// Test externalLabels is longer than labels.
+		{
+			labels:         labels.Labels{{Name: "c", Value: "d"}},
+			externalLabels: labels.Labels{{Name: "a", Value: "b"}, {Name: "e", Value: "f"}},
+			expected:       labels.Labels{{Name: "a", Value: "b"}, {Name: "c", Value: "d"}, {Name: "e", Value: "f"}},
+		},
 	} {
 		require.Equal(t, tc.expected, processExternalLabels(tc.labels, tc.externalLabels))
 	}