feat(subscriber): Add cfg console_without_tokio_unstable (#446)

Add cfg `console_without_tokio_unstable` for developers to turn off the
assertion on `tokio_unstable`. This is useful for non-tokio runtimes
which has `tokio-console` support.

console-subscriber/README.md

@@ -206,6 +206,13 @@ tracing_subscriber::registry()
 [`tokio-console`]: https://github.com/tokio-rs/console
 [Tokio]: https://tokio.rs
 
+### Using other runtimes
+
+If you are using a custom runtime that supports tokio-console, you may not need
+to enable the `tokio_unstable` cfg flag. In this case, you need to enable cfg
+`console_without_tokio_unstable` for console-subscriber to disable its check for
+`tokio_unstable`.
+
 ### Crate Feature Flags
 
 This crate provides the following feature flags and optional dependencies:

console-subscriber/src/lib.rs

@@ -267,7 +267,7 @@ impl ConsoleLayer {
         // depending on the build-time configuration...
         #![allow(clippy::assertions_on_constants)]
         assert!(
-            cfg!(tokio_unstable),
+            cfg!(any(tokio_unstable, console_without_tokio_unstable)),
             "task tracing requires Tokio to be built with RUSTFLAGS=\"--cfg tokio_unstable\"!"
         );