Implement global throttling

[daveallie]

Allow users to specify global_throttling, meaning all requests from an identifier to an endpoint with global_throttling enabled contribute toward the global_throttling_max.

Useful for app-wide throttling

[gottfrois]

Thanks for the pull request. I'm not sure about a global throttling capability since the purpose of this gem was really to allow "per endpoint" throttling capability.

[daveallie]

It's isn't on by default, it's opt in on both the app and individual endpoint levels, (i.e. if I wan't endpoint A and B to contribute to the same total).

No hard feelings if this doesn't get merged in just thought I would spend a couple of minutes knocking this up as it was something I was looking for when looking to use this gem.

[gottfrois]

Maybe we should allow to customize the key used to track stats per endpoints. This way, one could do something like this:

Grape::Attack.configure do |config|
  config.max = 1000
  config.per = 1.day
end

and on each endpoints:

throttle key: "my_app"
get 'foo' do
  ...
end

[daveallie]

Yeah I like that a lot more than the way I implemented it. It gives you the option to group endpoints in any way you want