Hey @Rodrigo_Arguello welcome and thanks for the question!
As far as I can tell, the collection name is always the value associated with the command name (i.e. bsonCommand[evt.CommandName]). Check out the Database Commands section of the MongoDB Manual for a more comprehensive answer.
As far as the actual CommandStartedEvent (and other command events), the information provided is specified in the Events API section of the MongoDB driver specifications. That specification doesn’t currently include collection name, but I can definitely see a use case for it.