code: def _before(self):
          
          # Don't trace excluded routes.
          if request.path in self.excluded_routes:
              request._tracy_exclude = True
              return
  
          request._tracy_start_time = monotonic()
          client = request.headers.get(trace_header_client, None)
          require_client = current_app.config.get("TRACY_REQUIRE_CLIENT", False)
          if client is None and require_client:
              abort(400, "Missing %s header" % trace_header_client)
  
          request._tracy_client = client
          request._tracy_id = request.headers.get(trace_header_id, new_id())
docs: Records the starting time of this reqeust.