FYI: input pipeline inspect endpoint

[Filip Filmar]

Hi folks.

Just to let you know, if you refresh and rebuild your Fuchsia source, you'll now be able to 

inspect the input event flow through the input pipeline.  The available metrics are rudimentary

at the moment: we count the number of events seen, and record the timestamps at which they

happened. We could add more if you have a use case.

Use case for this change are bugs such as fxbug.dev/81262 or fxbug.dev/82593 where input events stop being delivered.  Monitoring for changes in the input pipeline metrics enable us to

determine quickly whether an issue lies with the driver or with the input pipeline itself.

Usage details are in the appropriate README.md.

Cheers,
F