Rename metric worker base class

I have renamed the metric worker base class so it complies with the
naming convention of the other metric worker.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Refactor metric worker hierarchy

I have added a generic worker class which can be used for metrics which
do not need to be initialized. Further, I have implemented a bottom-up
structure. If there is a special metric worker class, instantiate it,
otherwise use the generic worker.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Move method for deriving metric worker class

Just moved the method for retrieving a specific worker class into the
base metric worker class.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Complete metric worker refactoring

Just had to adjust the metric worker classes to the new classes to
implement the bottom-up structure.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>