Ruby SDK requires Rack to be installed

[RyanGWU82]

The Ruby SDK requires the Rack HTTP web server to be installed. There are two problems here: First, rack isn't specified in the Gemfile. Second, rack is required even if you're just using readme/webhook which doesn't directly depend on Rack.

I think the second issue is because when you require readme/webhook, it loads lib/readme/webhook.rb. The first line of that file is:

metrics-sdks/packages/ruby/lib/readme/webhook.rb

Line 1 in 4d1f6b2

require 'readme/metrics'

This loads the entire readme/metrics package, and that needs Rack. But I think we have to do it this way so that we can "monkeypatch" the Readme module.

Two solutions that I think will help with all this:

1. Create some other overarching file that defines the Readme module, so that readme/webhook never needs to load readme/metrics. (My Ruby is pretty rusty so I'm not sure whether this even makes sense, but I think there's probably something we can do with an extra layer of indirection…)
2. Add rack to the Gemfile.