Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
* Implement xpack.monitoring.elasticsearch.collection.enabled setting
* Fixing line lengths
* Updating constructor calls in test
* Removing unused import
* Fixing line lengths in test classes
* Make monitoringService.isElasticsearchCollectionEnabled() return true for tests
* Remove wrong expectation
* Adding unit tests for new flag to be false
* Fixing line wrapping/indentation for better readability
* Adding docs
* Fixing logic in ClusterStatsCollector::shouldCollect
* Rebasing with master and resolving conflicts
* Simplifying implementation by gating scheduling
* Doc fixes / improvements
* Making methods package private
* Fixing wording
* Fixing method access
