In https://jira.mongodb.org/browse/SERVER-112507, we declared that the traditional static DocumentSource id allocation does not work for extensions as one DocumentSourceExtension may not be correlated to the same stage name as another DocumentSourceExtension. Thus, they need different ids.

However, upon further investigation, this isn't true. DocumentSource registration is done by stage name, meaning that $foo will have a different parser implementation than $bar, even if they are both DocumentSourceExtensions. The Id is necessary to connect the DocumentSource to the exec::agg ExtensionStage. As such, it's ok for the id to remain static as all DocumentSourceExtensions will have the same ExtensionStage implementation (get the _logicalStage from the DocumentSourceExtensionOptimizable and call compile() on it).