A session represents a conversation between a Dialogflow agent and an
end-user. You can create special entities, called session entities,
during a session. Session entities can extend or replace custom entity
types and only exist during the session that they were created for.
All session data, including session entities, is stored by Dialogflow
for 20 minutes. For more information, see the session entity guide
<https://cloud.google.com/dialogflow/docs/entities-session>__.
Required. Indicates whether the additional data should
override or supplement the custom entity type definition.
