IGNITE-13399 Fix access right issues in computation of system metrics

[Denis Mekhanikov]

Hi everyone!

I've prepared a PR for the following issue:
https://issues.apache.org/jira/browse/IGNITE-13399
Currently on Java 8 CpuLoad metric is reported to be equal to -1 when
running in embedded mode.

Could anyone take a look?
Thanks!

Denis