tracing: instrument mpsc channels

[domodwyer]

This PR adds instrumentation to bounded & unbounded mpsc channels, similar to the existing tracing for oneshot/mutexes/etc.

⚠️ potential deadlock ⚠️

Currently the primary consumer of this tracing would be the tokio-console (see feature request) however the console-subscriber crate will deadlock with this PR applied and potentially other tracing subscribers that also use an mpsc channel internally too. I'm not sure if this should be merged before the tracing/console-subscriber deadlock is fixed (I suspect not) but wanted to get a review on the changes, as well as sharing a reproducer. I'm more than happy to rebase this as needed until everything is ready to merge 👍

This PR copies the #[cfg(...)] approach of #4302, but I noticed the comment from @carllerche suggested cleaning this up - I'm more than happy to do this in a follow-up PR and clean up the existing tracing #[cfg(..)] too.

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• tracing: instrument mpsc channels (4526247)

  This commit adds tracing to both bounded and unbounded mpsc channels.
  
  Unbounded channels record the following state fields:
  
  	tx: number of items sent through the channel
  	rx: number of items received through the channel
  	values: number of items currently held in the channel
  	tx_handles: number of Sender handles for the channel
  	rx_dropped: true if the Receiver has dropped
  
  Bounded channels record the same fields as above, with the addition of the
  static "capacity" field set to the channel buffer size.
  

[Darksonn]

Thank you for the PR. Unfortunately, it looks like got lost. Since it has been so long, I'm going to close this, even though this is still something that I think would make sense to do. I'm not sure how you could fix the deadlock issue.