A Metric is a 'quantitative measure' of something for example the number of operations pending, number of operations
 per second being executed etc.
 Each metric is uniquely identified using a name. See 
getName().
 A metric gets its content from a probe. An probe can be one of the following:
 
     - a method with the 
Probe annotation 
     - a field with the 
Probe annotation 
     - a 
LongProbeFunction 
     - a 
DoubleProbeFunction 
 
 If the Metric is obtained before a probe is registered, the Metric is without probe. As long as no probe is available,
 every time the metrics needs a value, it will look up the probe in the 
MetricsRegistry.