message: Add support for "$duration" to event filter expressions
diff: @@ -913,6 +913,11 @@ const char *filter_get_keyval_func( const trace_event_t *event, const char *name
  snprintf_safe( buf, "%.6f", event->ts * ( 1.0 / MSECS_PER_SEC ) );
  return buf;
  }
  + else if ( !strcasecmp( name, "duration" ) )
  + {
  + snprintf_safe( buf, "%.6f", event->duration * ( 1.0 / MSECS_PER_SEC ) );
  + return buf;
  + }
  for ( const event_field_t &field : event->fields )
  {
  