зарегистрировать глобальное производное имя и определение метрики (register a global derived metric name and definition)
Предостережение (Caveat)
Derived metrics are not available when using pmFetchArchive(3) as
this routine does not use a target list of PMIDs that could be
remapped (as is done for pmFetch(3)).
There is no pmUnregisterDerived method, so once registered a
derived metric persists for the life of the application.
