Add diagnostics channel to layers

[Qard]

This is a proof-of-concept implementation of the diagnostics_channel feature I'm working on in Node.js core. The value here is for APM products to be able to capture changes to routing state through the lifecycle of a request. This should of course not be merged until the module exists in Node.js itself. I'm just creating this now to prove the value to userland and to make sure a more complex framework can benefit from it.

I'll leave this as a draft PR until diagnostics_channel lands in Node.js core.

[dougwilson]

Thank you so much @Qard ! Espeically if this can not be landed relatively soon, you should close this PR on this repo and instead open it on our upstream of the router: https://github.com/pillarjs/router . In the Express 5 branch, all of this code is deleted, so your changes would be lost if merged here. If your changes land in the router 1.x branch, they are automatically ported into the "legacy" Express 4.x code case, which is what your PR here is against.

[Qard]

Ah, cool. Didn't know about the backporting. I'll get this moved over. Thanks for the help! 👍

[Qard]

New PR opened here: pillarjs/router#96, closing this now. :)