Make inclusion of railtie opt-in

[ejholmes]

Closes #149

For those of us that want to use the dd-trace-rb gem without automatically including rails magic, this makes it easier. If people want the railtie, they can require 'ddtrace/rails' to get it.

This wouldn't be backwards compatible with existing installations, so maybe it would need to go in a 0.8 or 1.0 release.

[ufoot]

Interesting. I'm indeed slightly worried about introducing a backward incompatible change. Currently trying to figure out where we could explicitly disable any op based on app config (but AFAIR there's a chicken and egg problem as when you have the app config it's sometimes too late). Another simple option is to opt-out based on some env var. Note that in non-Rails context, require ddtrace just does nothing (as Rails::VERSION is not here). I think the case "I use Rails and use ddtrace but don't want ddtrace to connect to Rails" is not mainstream, so would advocate for an opt-out. But it's true Rails is really an exception, as almost all other components need to be explicitly monkey patched.

[ejholmes]

I hear you. An opt-out like how pry-rails does it would be alright, but would definitely prefer an explicit require 'ddtrace/rails' or a ddtrace-rails gem for the rails magic.

[ufoot]

#173 implements the opt-out strategy