Expose QOS settings for a topic

[Myzhar]

Feature request

Feature description

Since a subscriber can receive a topic message only if its QOS profile is compatible with the QOS profile of the publisher a method to retrieve this information from the available topics should be very useful.

Implementation considerations

I think that the command
$ ros2 topic info /topic
can be a good place to introduce this important feature.

Ref: https://answers.ros.org/question/304946/ros2-retrieving-qos-settings-for-a-topic/

[emersonknapp]

This feature now exists! @mjcarroll can you close it? I don't have the access to do so.