Demonstrating the inconsistent, confusing behavior of tracing_subscriber default init methods.

For all methods, compare what is output by the following, given different init conditions:

    tracing::debug!("debug message!");
    tracing::info!("info message!");
    tracing::error!("error message!");

1. Initialize with tracing_subscriber::fmt::init():

   $ cargo run --features init-function
   2025-06-30T19:54:28.027555Z  INFO binary_crate: info message!
   2025-06-30T19:54:28.027675Z ERROR binary_crate: error message!
   
   $ RUST_LOG=debug cargo run --features init-function
   2025-06-30T19:54:33.248545Z DEBUG binary_crate: debug message!
   2025-06-30T19:54:33.248623Z  INFO binary_crate: info message!
   2025-06-30T19:54:33.248637Z ERROR binary_crate: error message!
   

   Great! That's exactly what I expected. Default to info, override with RUST_LOG.

2. Initialize with tracing_subscriber::fmt().init():

   $ cargo run --features init-method
   2025-06-30T19:55:12.333057Z  INFO binary_crate: info message!
   2025-06-30T19:55:12.333123Z ERROR binary_crate: error message!
   
   $ RUST_LOG=debug cargo run --features init-method
   2025-06-30T19:55:18.745709Z  INFO binary_crate: info message!
   2025-06-30T19:55:18.745823Z ERROR binary_crate: error message!
   

   It does not obey RUST_LOG!. I have hit this footgun multiple times, and I know of many services I've deployed that aren't going to listen to RUST_LOG as a result.

3. Initialize with tracing_subscriber::fmt::init(), but with a transient dependency on the env-filter feature:

   $ cargo run --features init-function,transient-env-filter
   2025-06-30T19:57:07.640891Z ERROR binary_crate: error message!
   
   $ RUST_LOG=debug cargo run --features init-function,transient-env-filter
   2025-06-30T19:57:14.889001Z DEBUG binary_crate: debug message!
   2025-06-30T19:57:14.889113Z  INFO binary_crate: info message!
   2025-06-30T19:57:14.889121Z ERROR binary_crate: error message!
   

   What?! All the sudden the same exact code from #1, which worked as expected, suddenly stops working by adding an unrelated dependency. We add the lib-crate dependency, which has 0 lines of code, but simply because it also depends on tracing-subscriber and the env-filter flag our logging breaks.