In #101587, Kibana is going to use a custom kbn-context header to pass execution context during a request from the Kibana browser app to the Kibana server.
With elastic/apm-agent-rum-js#1040, we should be able to remove the custom header and switch to APM RUM agent API that will allow associating context metadata with an APM transaction. APM RUM agent will take care of serialization and passing context via proper tracing headers.