[RFC 24](https://github.com/googlecartographer/rfcs/blob/master/text/0024-monitoring-ros.md)
Public API:
- adds `cartographer_ros::metrics::FamilyFactory`
- compatible with `::cartographer::metrics::RegisterAllMetrics`
Public RPC interface:
- adds the ROS service `/read_metrics`
- response contains the latest values of all available metric families