Add watcher to values retrieved via config sources (#2803)
Add watcher to values retrieved via config sources (#2803)

* Add a watcher for configuration source objects

* Rename Apply to Retrieve and change return type

* Add Watcher and a RetrieveEnd method

* Remove Watcher interface in favor of a method

* Add ErrSessionClosed

* Move var to top
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib