Improve faraday configuration

[p-lambert]

This PR let's faraday be customized globally and on a per-connection basis.

Setting a global service_name

Datadog.configure do |c|
  c.use :faraday, service_name: 'http'
end

Setting a connection-specific service_name

connection = Faraday.new('http://example.com') do |builder|
  builder.use(:ddtrace, service_name: 'example-request')
end

[palazzem]

Just a nitpick, then the approach is exactly what we need.

[palazzem]

shouldn't it be faraday.request or similar? SERVICE is pointing to faraday-request and it's not a SERVICE but an Operation Name.