cmake/target.cmake defines macros for the supported platforms, this
commit changes predefined system macros to our own macros.
__linux__ --> OS_LINUX
__APPLE__ --> OS_DARWIN
__FreeBSD__ --> OS_FREEBSD
* New metrics provider (Procfs) + Refactored TasksStatsCounters
* Trivial statless test that ProcFS is provided
* Trivial perf test for ProcfsMetricsProvider
Co-authored-by: alexey-milovidov <milovidov@yandex-team.ru>