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:
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.