Support disabling the agent per config

[slhck]

Is your feature request related to a problem? Please describe.

I want to disable the APM agent in a particular Rails environment, or via an environment variable setting, to dynamically turn on/off APM reporting. In particular I do not want instrumentation in the Rails test environment.

Describe the solution you'd like

Provide a config option, just like the Node.js version has:

A boolean specifying if the agent should be active or not. If active, the agent will instrument incoming HTTP requests and track errors. Normally you would not want to run the agent in your development or testing environments. If you are using the NODE_ENV environment variable, you can use this to determine the state:

Describe alternatives you've considered

Not loading the Gem, but it is automatically loaded when specified in the Gemfile.

[slhck]

I was able to work around it by specifying require: false in the Gemfile, and then requiring the Gem only in config/environments/production.rb. That, however, makes it difficult to use ElasticAPM classes throughout the code, since they will have to be wrapped in a if Rails.env.production? statement (otherwise you'll get a class not found error).