Where are ros2 message ostream operators for c++ logging?

edit

UPDATE: It seems the original method below is now deprecated in Humble. You should now use geometry_msgs::msg::to_yaml() instead of rosidl_generator_traits::to_yaml()

warning: ‘std::string rosidl_generator_traits::to_yaml(const Pose&)’ is deprecated: use 
geometry_msgs::msg::to_yaml() instead [-Wdeprecated-declarations]

So, the answer for Humble is

#include <rclcpp/rclcpp.hpp>
#include <geometry_msgs/msg/pose_stamped.hpp>
[...]
geometry_msgs::msg::PoseStamped poseMsg = [...];
RCLCPP_INFO_STREAM(get_logger(), "posevalue:" << geometry_msgs::msg::(poseMsg));
[...]

ORIGINAL: I had the same issue... here's how to fix it:
use rosidl_generator_traits::to_yaml(poseMsg) instead of just poseMsg

#include <rclcpp/rclcpp.hpp>
#include <geometry_msgs/msg/pose_stamped.hpp>
[...]
geometry_msgs::msg::PoseStamped poseMsg = [...];
RCLCPP_INFO_STREAM(get_logger(), "posevalue:" << rosidl_generator_traits::to_yaml(poseMsg));
[...]

I dug through the generated message header files and found it.