Add HTTP multiplexing

[delner]

As a PR that replaces #882 (incorporating @stormsilver's original contributions; thank you!), this pull request aims to add "multiplexing" to the other HTTP client libraries instrumented.

In practice, "multiplexing" allows users to configure different tracer settings for different URLs. The most common reason why you might prefer to do this is to mark some outgoing HTTP calls as belonging to one service, while marking others as belonging to another.

e.g.

Datadog.configure do |c|
 # Assign all outgoing Net::HTTP requests to amazonaws.com to the 'aws' service
  c.use :http, describe: /amazonaws\.com/ do |http|
    http.service_name = 'aws'
  end
end

It also adds split_by_domain options which assigns the FQDN of the request URL as the service name.

These changes affect the following libraries:

• Ethon
• Excon
• Faraday
• HTTP

[delner]

Changes are working, but might want to do a little refactoring/testing before merging. Could use a review from you as well @marcotc.

[marcotc]

It looks very good overall! 👍

[marcotc]

🔥