Support rcl_interfaces MetricsMessage

[dabonnie]

Description

To support the ROS2 Topic Statistics feature, support for the statistics_msgs::msg::MetricsMessage is needed. This is the same message found in this branch but the dependency has simply moved to another package (now core ROS2).

Test Plan

• Manual testing
  • ensure messages can be published to cloudwatch
• Unit test for new classes
  • add a unit test, similar to this functionality to send the new message

Documentation Plan

• README.md update
• wiki update

Release Plan

• check in PR
• bloom release to support Foxy

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

• Original PR