Ability to resize the async main()'s thread pool from within

[Pr0methean]

Is your feature request related to a problem? Please describe.
On AWS's m7g.16xlarge instances, the detected number of available CPU cores is sometimes 63 rather than 64, which is the actual number of VCPUs available. This slows down sufficiently parallel work.

Describe the solution you'd like
A way for a #[tokio::main] async fn main() method to update the desired number of threads in the pool where it was running. The desired and actual numbers of threads would be stored in atomics with at least Ordering::Acquire for increases and Ordering::Release for decreases (and I'm not 100% sure even that is enough; it's possible Ordering::SeqCst would be needed throughout). Whenever a thread was between tasks and the actual number of threads exceeded the desired number, that thread would terminate. Whenever the desired number of threads was less than the actual number, all the existing threads were busy, and a new task was submitted, that task would run right away on a new thread. (In other words, the actual number of threads would be able to exceed the desired number, but not for longer than m + n + O(1) where m and n were the top two task durations.)

Describe alternatives you've considered
When I was using Rayon rather than Tokio, I could call the following from the sync main():

    let mut global_builder = ThreadPoolBuilder::new().use_current_thread();
    match available_parallelism() {
        Ok(parallelism) => {
            let adjusted_parallelism = parallelism.get() + 1;
            if adjusted_parallelism.count_ones() <= 1 {
                warn!("Adjusting CPU count from {} to {}", parallelism, adjusted_parallelism);
                // Compensate for missed CPU core on m7g.16xlarge
                global_builder = global_builder.num_threads(adjusted_parallelism);
            } else {
                info!("Rayon thread pool has {} threads", parallelism);
            }
        }
        Err(e) => warn!("Unable to get available parallelism: {}", e)
    }
    global_builder.build_global()?;

Additional context
Add any other context or screenshots about the feature request here.

[maminrayej]

I think something like this can be implemented by the user after #6341 is implemented.

[Darksonn]

Closing as duplicate of #6341.

If you just need to set this in the beginning of main, then you may want to explicitly use the runtime builder rather than the #[tokio::main] macro and set the right number from the beginning.