Right now there's a [decorator](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/route/v3/route_components.proto#envoy-v3-api-msg-config-route-v3-decorator) api in route, which seems related to tracing but not under [tracing](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/route/v3/route_components.proto#config-route-v3-tracing). 1. should we move it under tracing? 2. it would be better if it support substitution formatter for better customization. cc @wbpcode
