my_gem.rb

# Inside the gem
class ActionController::Base
  def self.check_webhook_signature(key, *args)
    before_filter lambda { |controller| controller.check_webhook_signature(key, controller)}, *args
  end

  def check_webhook_signature(key, controller)
    # Do the work
  end
end

webhook_controller.rb

# Gem was included through Gemfile

class WebhookController < ApplicationController
  # Gem provides a Class method to ActionController::Base that inserts a before_action
  check_webhook_signature "MyKey"
  
  def index
    # I can do my thing here
  end
end

Pros:

• Still one line
• Puts the behaviour in the same place as the rest of the code
• Does not require that every request be regex'ed to see if the middleware needs to do its thing

Cons:

• Rails specific