Look into an API for finding ActiveSupport::Notifications subscribers

[kaylareopelle]

There's a decade-old TODO in our notifications_subscriber:

newrelic-ruby-agent/lib/new_relic/agent/instrumentation/notifications_subscriber.rb

Lines 19 to 23 in 90b5487

	def self.find_all_subscribers
	# TODO: need to talk to Rails core about an API for this,
	# rather than digging through Listener ivars
	instance_variable_names = [:@subscribers, :@string_subscribers, :@other_subscribers]
	all_subscribers = []

Let's look into whether there is an API for finding all subscribers as an alternative to this approach. If there isn't one, consider:

• Creating an issue for the Rails core team to address it
• Submitting a PR to Rails that creates an API to access all subscribers

Remove the TODO when finished.

Whatever we come up with should be OTel-informed.

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-4113