message: docs: add tracing-elastic-apm to related crates
  ## Motivation
  From the readme
  > (if you're the maintainer of a tracing ecosystem crate not in this list, please let us know!)
  ## Solution
  Added links to tracing-elastic-apm.
diff: @@ -399,6 +399,7 @@ are not maintained by the `tokio` project. These include:
  - [`diesel-tracing`] provides integration with [`diesel`] database connections.
  - [`tracing-tracy`] provides a way to collect [Tracy] profiles in instrumented
  applications.
  +- [`tracing-elastic-apm`] provides a layer for reporting traces to [Elastic APM].
  (if you're the maintainer of a `tracing` ecosystem crate not in this list,
  please let us know!)
  @@ -426,6 +427,8 @@ please let us know!)
  [`diesel-tracing`]: https://crates.io/crates/diesel-tracing
  [`tracing-tracy`]: https://crates.io/crates/tracing-tracy
  [Tracy]: https://github.com/wolfpld/tracy
  +[`tracing-elastic-apm`]: https://crates.io/crates/tracing-elastic-apm
  +[Elastic APM]: https://www.elastic.co/apm
  **Note:** that some of the ecosystem crates are currently unreleased and
  undergoing active development. They may be less stable than `tracing` and
  