opentelemetry_req: Don't assume request.headers shape

[wojtekmach]

I'm planning to switch Req headers to be maps. This patch will work on either representation. After we switch to maps, Req.Request.put_header/3 should be plenty fast, however I'd be happy to send another patch that updates the headers in bulk if that'd be more desirable.

cc @bryannaegele

[wojtekmach]

I just noticed there's a couple ongoing PRs:

• Allow to configure which headers should be used as span attrs #184 (cc @albertored)
• Add tests and tweak behavior of opentelemetry_req #194 (cc @andrewhr, thanks for pointing out possible conflicts!)

I'm happy to wait until they are addressed and rebase on top. :)

Just FYI, the :headers change would only happen in Req v0.4.0 which is planned to happen in the next couple of weeks. Ideally :opentelemetry_req would have depended on {:req, "~> 0.3.0"}, that is, projects using both dependencies would not have picked up req v0.4.0 when it comes out. But I think it's gonna be ok, if we can get a new opentelemetry_req version with a patch like this one, I'm happy to mention the need to upgrade in my Req v0.4 changelog. Sorry for the extra hassle, if there's anything I can do to smooth the transition on req or opentelemtry_req, please let me know!

[bryannaegele]

@wojtekmach and @andrewhr I'm going to go with this PR with the tests as-is in here so we can get a release out to support >= 0.4