[BZ 1072543] Unable to collect metrics from EAP 6 datasources when a system properties is used in configuration

max-pool-size and min-pool-size metrics got '_expr:' prefix so they're
recoginzed by common code to resolve a possible expression.
BaseComponent.getValues has been changed in a way, that we have more
granular method called BaseComponent.getMetricValue which handles single
metric request. This method returns new enum called ReadMetricResult which
just represents a return code (if not Success, we may want to know logical
reason why metric was not collected)

Asking for review, because I changed (hopefully did not break) base method being used all over plugin