ObservableInstrument<T>
ObservableInstrument{T} is the base class from which all metrics observable instruments will inherit from.
Create the metrics observable instrument using the properties meter, name, description, and unit.
All classes extending ObservableInstrument{T} need to call this constructor when constructing object of the extended class.
Observe() fetches the current measurements being tracked by this instrument. All classes extending ObservableInstrument{T} need to implement this method.