Add Statsd::RackEndpoint

[jamesarosen]

This endpoint makes it easy for sites to collect client-side stats. It exposes (most of) the functionality of the existing Statsd client object as a Rack endpoint.

It is not automatically required when loading the library, nor did I add Rack as a runtime dependency. (I would have added it as an optional dependency if Rubygems supported that concept.)

[jamesarosen]

1. It might be worth mentioning in the docs for this class that it is highly recommended that the front-end code use the sample_rate parameter and send only a portion of the requests to reduce HTTP traffic.
2. If it would be useful, I'm happy to implement batch support via POST bodies. That would complicate the class a bit, but it might be worth it to make it easier to further reduce HTTP traffic.

[jamesarosen]

I'm also happy to add a reference client implementation. I'd like to keep it as framework-agnostic as possible, but I also don't want to reimplement $.ajax and _.debounce. Perhaps something like

var $ = window.$ || {},
    _ = window._ || {},
    ajax = $.ajax || function() { throw "Please define StatsdClient.ajax"; },
    debounce = $.debounce || _.debounce || function() { throw "Please define StatsdClient.debounce"; };

function StatsdClient() {
  // ...
}
StatsdClient.ajax = ajax;
StatsdClient.debounce = debounce;

[reinh]

I think this would be better as a separate gem is it isn't core functionality and it's easy to make it a separate gem.

[jamesarosen]

OK. I'll work on that.

[reinh]

@jamesarosen Let me know when it's available and I'll link it up in the README! Thanks!