[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
This will allow us to keep way less information in memory for SLAMing and fixes crashes related to multi trajectory (e.g. this fixes#413).
It also means that to get to an X-Ray or a map, users need to run the asset writer now after SLAMing, which is inconvenient.
Remove dependency on YAML and delete more dead code.
When receiving a FinishTrajectory call, the Cartographer nodes catches up on
computing loop closures, runs a final optimization, and when in 2D writes out
a PGM map similar to map_saver.
The 'version_' member of a DrawableSubmap is now properly initialized.
The redundant 'submap_id' in the SubmapQuery response is removed.
The missing slot for reset() is added.