Steps towards the use of logging API and minimal logger severity configuration [limitations in square brackets].

Minimal functionality

• Logger objects in rclpy Expose logging macros in rclpy rclpy#103
• rclpy nodes have loggers associated with them [naming conventions hard-coded] Give nodes a logger rclpy#148
• C++ macros wrapping rcutils [specifying logger name instead of logger object] Implement rclcpp-specific logging macros [taking name not object] rclcpp#389
• Update demos to use logging [callbacks need access to node logger, C++ passing names not logger object] Replace print() in python demos with logger calls demos#190, Use logging macros [logger name instead of object] demos#191
• ROS 2 core using debug logging [no subloggers of nodes' loggers] Add debug logging rcl#187

Demo/documentation

• Demo of service that sets logger level at runtime for logger name Add demo for logging + logger config demos#194 https://github.com/ros2/ros2/wiki/Logging-and-logger-configuration
• Wiki page on logger configuration https://github.com/ros2/ros2/wiki/Logging

C++ logger object

• C++ macros taking logger object, factory functions Add Logger class and give one to nodes rclcpp#411 Swap to using logger object for logging macros demos#200
• Node logger name added to nodes in rcl, wrapped in python Store logger name associated with node rcl#212, Get node's logger name from rcl rclcpp#433, Get node's logger name from rcl rclpy#174
• ROS 2 core using subloggers from node's logger

Enhancements

• Configuration of non-node loggers' context
• Improve ease-of-use of accessing node logger from callbacks
• Wrapping of "throttled" filter in C++ macros (done in Enable throttling logs rclcpp#879)
• Unique context of e.g. "once" for each logger object