message: tracing: fix lib.rs docs links
  Some of the links to external crates in the `lib.rs` docs
  were missing. This fixes them.
diff: //! [`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
  +//! [`tracing-etw`]: https://github.com/microsoft/tracing-etw
  +//! [ETW]: https://docs.microsoft.com/en-us/windows/win32/etw/about-event-tracing
  //! [`tracing-fluent-assertions`]: https://crates.io/crates/tracing-fluent-assertions
  //!
  //! <div class="example-wrap" style="display:inline-block">
  