feat: Tracer in Transport

[shaxbee]

Add Tracer argument to nethttp.Transport.
Fixes #29

[shaxbee]

Hold on merging, there is sp state in Tracer, need to fix this to avoid race condition.

[shaxbee]

Fixed and ready for review.

I would suggest small refactoring of Tracer clientOptions - instead making them public structure with simple public fields, as there is unnecessary overhead of applying functional options on each call plus it cannot be used without dedicated constructor. Let me know if i can make those changes.

[yurishkuro]

it would be a breaking change. Rather than refactoring it would have to be differently named constructor function

[shaxbee]

Alright, would current change be acceptable then?

[yurishkuro]

What happens to the root span created in start() function if the user does not call finish()?

	req, ht := nethttp.TraceRequest(tracer, req)
	defer ht.Finish()

Also, the PR needs unit test for the new mode and update to the docs showing the alternative usage.

[wpjunior]

@shaxbee and @yurishkuro I had wrote a wrapper to put tracer inside the transport at tsuru.io.

You could see the code here: https://github.com/tsuru/tsuru/blob/main/net/opentracing.go#L29

I hope that will help to achieve this solution.

[yurishkuro]

closing old/stale PR