Concurrent Ruby: Stop assuming a version number is available where we expect it to be

[fallwith]

Currently the agent will assume that the Concurrent::VERSION constant exists as long as the Concurrent constant is defined. We perform a version check to ensure that version 1.1.5 or higher is in play before instrumenting the gem.

We have learned from a customer with with Concurrent Ruby version 1.1.6 that it is possible for Concurrent Ruby to satisfy the Concurrent check but fail to provide Concurrent::VERSION. This results in the Concurrent Ruby instrumentation erroring out. Given that each instrumentation's routines are siloed, the only real ill effect is error messages hitting the agent log.

We should confirm that Concurrent::VERSION exists prior to using its value in a numeric comparison with our minimum supported gem version.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-232908

[fallwith]

Here's the relevant observed newrelic_agent.log message:

DEBUG : Using auto configuration value for concurrent_ruby to configure instrumentation
ERROR : Error while detecting concurrent_ruby:
ERROR : NameError: uninitialized constant Concurrent::VERSION
DEBUG : Debugging backtrace:
$HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/agent/instrumentation/concurrent_ruby.rb:14:in `block (2 levels) in <top (required)>'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:126:in `block in check_dependencies'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:124:in `each'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:124:in `all?'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:124:in `check_dependencies'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:64:in `dependencies_satisfied?'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:28:in `block in detect!'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:27:in `each'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/dependency_detection.rb:27:in `detect!'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/control/instrumentation.rb:64:in `_install_instrumentation'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/control/instrumentation.rb:47:in `install_instrumentation'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/control/private_instance_methods.rb:39:in `init_instrumentation'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/new_relic/control/instance_methods.rb:75:in `init_plugin'
  $HOME/.asdf/installs/ruby/2.5.7/lib/ruby/gems/2.5.0/gems/newrelic_rpm-9.7.1/lib/newrelic_rpm.rb:40:in `<top (required)>'

[fallwith]

The version constant Concurrent::VERSION exists in the oldest available version of the gem, v0.7.0. So this doesn't appear to be a version issue.

[fallwith]

We don't have a repro, but #2466 is a bit more paranoid with an extra defined? check.