Move Internal Collection stack monitoring behind a feature flag

[robbavey]

There are currently multiple ways of monitoring Logstash, with internal collection, metricbeat and agent-driven monitoring available for users.

We believe that our users are best served by using agent-driven monitoring, and we would prefer to steer our users away from using internal collection, which has been deprecated since 7.9.0.

To that end, we will move internal collection behind a "feature flag", which will be disabled by default in 9.0. This provides a push to our users towards modern monitoring, while not blocking upgrades for users who still require it.