docs: add tracing-elastic-apm to related crates (#1146)

## 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.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

README.md

@@ -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

tracing/src/lib.rs

@@ -722,6 +722,7 @@
 //!  - [`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 listed above,
 //! please let us know! We'd love to add your project to the list!
@@ -747,6 +748,8 @@
 //! [`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
 //!
 //! <div class="information">
 //!     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>