Enable sensor metrics on example worlds

[chapulina]

🎉 New feature

Requires gazebosim/gz-sensors#146

Summary

Enable the new sensor metrics for the example worlds to give the feature more visibility.

Test it

For example:

ign gazebo -r src/ign-gazebo/examples/worlds/sensors_demo.sdf 

And on another terminal you can check the performance metrics topics:

$ ign topic -l | grep performance
/camera_alone/performance_metrics
/depth_camera/performance_metrics
/lidar/points/performance_metrics
/rgbd_camera/performance_metrics

And echo one of them:

$ ign topic -e -t /camera_alone/performance_metrics
...
name: "cameras_alone::link::cameras_alone"
real_update_rate: 30.017970558254408
sim_update_rate: 30.303030303032418
nominal_update_rate: 30
...

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

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[francocipollone]

LGTM!

[chapulina]

@osrf-jenkins run tests please

[codecov]

Codecov Report

Merging #982 (1f5a207) into ign-gazebo3 (c625586) will increase coverage by 0.04%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo3     #982      +/-   ##
===============================================
+ Coverage        77.79%   77.84%   +0.04%     
===============================================
  Files              221      221              
  Lines            12687    12687              
===============================================
+ Hits              9870     9876       +6     
+ Misses            2817     2811       -6     

Impacted Files	Coverage Δ
src/SimulationRunner.cc	94.49% <0.00%> (+1.06%)	⬆️

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