Instrument vm service extensions in the gallery (#17977)

https://github.com/flutter/flutter/issues/17956

* Add a `debugInstrumentAction()` method that will wrap an action
  in a stopwatch and print the time it took to run the action.
* Add a global `debugInstrumentationEnabled` that will control
  whether `debugInstrumentAction()` does anything (even in debug
  builds).
* Add some basic instrumentation to `registerServiceExtension()`
* Temporarily enable the `debugInstrumentationEnabled` flag in the
  Gallery to give us better visibility into what's happening in
  https://github.com/flutter/flutter/issues/17956