code: func (o *ClientOptions) SetCredentialsProvider(p CredentialsProvider) *ClientOptions {
  	o.CredentialsProvider = p
  	return o
  }
docs: // SetCredentialsProvider will set a method to be called by this client when
  // connecting to the MQTT broker that provide the current username and password.
  // Note: without the use of SSL/TLS, this information will be sent
  // in plaintext accross the wire.