metadataSendMode  
  This determines whether metadata is sent in requests to the custom provider.
off will not send any metadata. Payload will look like { messages }
variable will send assistant.metadata as a variable on the payload. Payload will look like { messages, metadata }
destructured will send assistant.metadata fields directly on the payload. Payload will look like { messages, ...metadata }
Further,
variable and destructured will send call, phoneNumber, and customer objects in the payload.
Default is
variable.