#1078 [HHQ-29] Better caching of MySQL metrics

[Hyperic Team]

http://jira.hyperic.com/browse/HHQ-29
The MySQL plugin does not do any caching of values from the 'SHOW' queries causing the query to be run for each invocation of getValue(). The results in inaccurate statistics for the 'Questions' and 'Questions per Minute' metrics.