The following bit of the README is wrong:

Add each applicable instrumentation to your application's Gemfile. For
example, this is how you add Rails and Redis:

$ gem 'signalfx-rails-instrumentation'
$ gem 'signalfx-redis-instrumentation'

That is mixing up Ruby with Bash.

Correct Bash: this is how you add Rails and Redis:

$ bundle add signalfx-rails-instrumentation
$ bundle add signalfx-redis-instrumentation

Correct Ruby in Gemfile:

gem 'signalfx-rails-instrumentation'
gem 'signalfx-redis-instrumentation'

Have to pick one, the other, or both, but they can't be merged!