message: docs: add blog posts as external resources
  As requested at
diff: @@ -448,6 +448,7 @@ Tracing.
  * [Diagnostics with Tracing][tokio-blog-2019-08] on the Tokio blog, August 2019
  * [Production-Grade Logging in Rust Applications][production-logging-2020], November 2020
  +* [Custom Logging in Rust using `tracing` and `tracing-subscriber`, part 1][custom-logging-part-1] and [part 2][custom-logging-part-2], October 2021
  [tokio-blog-2019-08]: https://tokio.rs/blog/2019-08-tracing/
  @@ -463,6 +464,8 @@ Tracing.
  [rusty-days-2020-08-video]: https://youtu.be/HtKnLiFwHJM
  [rusty-days-2020-08-slides]: https://docs.google.com/presentation/d/1zrxJs7fJgQ29bKfnAll1bYTo9cYZxsCZUwDDtyp5Fak/edit?usp=sharing
  [production-logging-2020]: https://medium.com/better-programming/production-grade-logging-in-rust-applications-2c7fffd108a6
  +[custom-logging-part-1]: https://burgers.io/custom-logging-in-rust-using-tracing
  +[custom-logging-part-2]: https://burgers.io/custom-logging-in-rust-using-tracing-part-2
  Help us expand this list! If you've written or spoken about Tracing, or
  know of resources that aren't listed, please open a pull request adding them.
  