Document new tracing feature

[crandmck]

When expressjs/express#2437 is merged, we need to add docs for the feature.

[frankrousseau]

How do you want to represent this doc? By adding app.instrument and res.trace to api doc or by writing a guide dedicated to tracing?

[crandmck]

@frankrousseau Ideally, both.

Bare minimum is the API doc, but we need a guide to really explain how to use it. If you can provide the basic information, I can edit, improve, format, etc. Here's a suggested outline:

• What's the purpose of the feature; common use cases, etc.
• Walk through and explain a simple example.
• (Optional) More complex example.

LMK if you want me to stub it out for you, and then you could just edit the markdown files.

[hacksparrow]

Best to work on it after the PR lands. The PR is still in the process of being smoothened out.

[frankrousseau]

@dougwilson said that the PR landing requires to have an open PR on the documentation. I will open it, so we can discuss about the canevas. We will be able to change the code examples after.

[crandmck]

It seems that this work will be moved into a separate module. I'll leave this issue open to track the doc work for that.

[crandmck]

I'm cleaning up old issues, and I don't see that there's any immediate doc work to do here...

If https://github.com/frankrousseau/express-tracer ever gets pulled into the expressjs org, then we can work on and incorporate docs for it.

Meantime, I'll close this.