* Micrometer instrumentation, part 2
* use double counter instead of gauge in FunctionTimer
* code review suggestions
* fix errorprone
* rebase and fix compilation failure
* Micrometer bridge instrumentation
* gauges with the same name and different attributes
* weak ref gauge
* one more test
* disable by default + muzzle
* code review comments
* log one-time warning
* make AsyncInstrumentRegistry actually thread safe
* code review comments
* one more minor fix