Create simple HTTP-logging middleware

[JoelQ]

🧰 What's being changed?

This creates a simple middleware that logs requests received to the
given logging API endpoint in the format: "GET http://example.com".

The actual Readme.io payload format is more complicated than this. We
will add support for it in a follow-up commit.

🧪 Testing

There is an automated test using Rspec and Rack::Test. We also tested manually with a simple Rack application that acts as both the client API and the Readme API:

# config.ru
$LOAD_PATH << File.expand_path("../path/to/metrics-sdks/packages/ruby/lib", __FILE__)
require "readme/metrics"

class ApiApp
  def call(env)
    puts "=== API ==="
    [200, {}, ["Ok"]]
  end
end

class ReadmeApp
  def call(env)
    puts "=== README ==="
    puts env['rack.input'].read
    [200, {}, ["Ok"]]
  end
end

map "/readme" do |builder|
  run ReadmeApp.new
end

map "/api" do
  use Readme::Metrics, "http://localhost:9292/readme"
  run ApiApp.new
end

Run this on the command-line with

$ rackup

Then try to hit one of the "client API" endpoints:

$ curl http://localhost:9292/api/path/to/thing

In the server logs of the rack application, you should expect to see:

[erunion]

lgtm as a solid framework for the coming work!

[joshuaclayton]

Couple minor/perhaps anticipatory comments around future changes, but this looks great!