HTTP adapter renamed to :http

[marcotc]

2.0 Upgrade Guide notes

In 1.x:

c.tracing.transport_options = proc do |t|
  t.adapter :net_http
end

In 2.0:

c.tracing.transport_options = proc do |t|
  t.adapter :http
end

Motivation:

The fact that the HTTP transport currently uses Net::HTTP is an implementation detail.

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[ivoanjo]

Hey, I think PR's upgrade notes description is off, since c.tracing.transport_options has been removed in #3350 🤔

[ekump]

Hey, I think PR's upgrade notes description is off, since c.tracing.transport_options has been removed in #3350 🤔

@ivoanjo is correct. Users can no longer explicitly set an adapter anymore.

[ekump]

Not to be pedantic, but I wonder if http is accurate enough since UDS also technically uses http? Perhaps sticking with http is necessary for consistency with other tracers and other datadog docs, so not blocking an LGTM from me.

[marcotc]

This change is not really important, so I'll just remove the possibly inaccurate comment instead #3411