Use a single publisher for a node's Topic Statistics

[dabonnie]

Description

If a node has multiple subscriptions, when Topic Statistics is enabled each subscription will have its own publisher. Instead a node should have a single publisher that is spun up when a node's subscription topic statistics are enabled and torn down when all the node's subscriptions topic statistics are disabled.

This issue came out of the review process for the Topic Statistics feature in ROS 2. When implementing this story please consider the following questions:

• is using a single publisher a worthwhile optimization? (e.g. verify a PoC with performance testing)
• is a single publisher a feasible use case? What if a user wants to use different topic names of publish periods per subscription?
• should publishers be shared given their configuration?

Test Plan

• Manual testing on all relevant platforms
• Unit test for new classes
  • new unit test for multiple subscriptions per node to ensure proper bringup / teardown of publisher
• Run ROS2 CI Job for relevant platforms and packages

Documentation Plan

• wiki update
  • update concepts with new behavior

Release Plan

• check in PR

Acceptance Criteria

• Code has been implemented, reviewed and merged.
• Test plan has been completed
• Release plan has been completed

Once all above items are checked, this story can be moved to done.

Resources

• ROS 2 Topic Statistics Meta Ticket