Report approximate size when accurate size is disabled

Disabling the accurate size computation with accurate-size="false makes the size/currentNumberOfEntries statistics report a value -1.

Since ISPN-13421 added approximate size metrics, we could make size/currentNumberOfEntries report the approximate size when the accurate size computation is disabled.

This way, the console and in any custom grafana dashboards would keep working even with accurate size disabled, but without overloading the servers with the accurate size computation.