At various places in the VM we query basic metaspace statistics (usually reserved+committed+used) by querying the individual values one after the other; this is inherently racy. Therefore we see rare intermittent errors in code which afterwards tests consistency of these values (e.g. JDK-8151460, JDK-8237872). 

Provide an API which returns consistent values and use them in these places. Affected are:

- Metaspace Perf Counters (MetaspaceCounters::update_performance_counters())

- NMT reporting (MemSummaryReporter::report_metadata)

and potentially others (see also JDK-8251392).