meta-ticket.

First question: which usage do we want with respect to the severity:

1. loginfo(...) (like in ROS 1)
2. log(..., severity=INFO)

Second question: when considering the other logging variations, which do we want?

1. log_info_throttle_named(...) (like the C usage)
2. log_info(..., throttle_duration=1.0, name='my_name') (more pythonic IMO)