remove jmx_enabled option that causes memory leak

[nicobrevin]

I've noticed that using the jmx_enabled option causes a memory leak. Each request creates a new JmxReporter, which is registered with the MetricsRegistry and never removed.

This commit takes away the jmx_enabled option in exchange for helper methods that let you set up a default jmx reporter during rack up or however you initialise your application.

[nicobrevin]

I've got a manual test case to reproduce, just in case you're interested:

# memory-leak.rb
require 'rubygems'
require 'rack'

require 'slf4j-api'
require 'metrics-core'
require 'metrics-annotation'

require 'jruby-rack-metrics'

leaky = true

if ! leaky
  com.yammer.metrics.reporting.JmxReporter.startDefault(com.yammer.metrics.Metrics.defaultRegistry)
end

app = Rack::Builder.new do
  use JrubyRackMetrics::Monitor, :jmx_enabled => leaky
  run (proc do |env|
      # the more possible statuses, the more metrics in each jmxreporter, so                                                                                                                                                       
      # the quicker this will die                                                                                                                                                                                                  
      possibles = [200, 302, 404, 503]
      status = possibles[rand(possibles.length)]
      [status, {"Content-Type" => "text/html"}, "Hello Rack!"]
  end)
end

Rack::Handler.get('webrick').run(app)

Run this like so:

jruby -J-Xmx64m -Ilib -Ipath/to/jars memory-leak.rb

And in another window, run apache bench:

ab -n 100000 -c 10 http://localhost:8080/

Once it is running, hook up jconsole and watch memory usage increase, response times go down and eventually it will ground to a halt. Switch leaky to false and memory usage stays steady.