libsysprof-capture: add metadata frame type

This will allow sources and aids to discover information about what was
done between capture loading and saving.

src/libsysprof-capture/sysprof-capture-cursor.c

@@ -159,6 +159,10 @@ sysprof_capture_cursor_foreach (SysprofCaptureCursor         *self,
           delegate = READ_DELEGATE (sysprof_capture_reader_read_counter_set);
           break;
 
+        case SYSPROF_CAPTURE_FRAME_METADATA:
+          delegate = READ_DELEGATE (sysprof_capture_reader_read_metadata);
+          break;
+
         default:
           if (!sysprof_capture_reader_skip (self->reader))
             return;