incremental_statistics: pass parameters by const& to the update method.
metrics: counter's deltas incremental statistics fixup.

Decreasing deltas go to the detlas statistics as negative values.
metrics: pass parameters to counter's methods by const&.
metrics: pass parameters to gauge's methods by const&.
metrics: initialization of timer's static member DEFAULT_INSTANCE_ID has been moved out to the source file.
metrics: pass parameters to timer's methods by const&.
events: pass parameters to counter's event functions by const&.
tests: unnecessary forward declarations have been removed from counter_events test.
measuring_points: pass parameters to counter's measuring points by const&.
measuring_points: pass parameters to gauge's mesauring points by const&.
measuring_points: pass parameters to timer's measuring points by const&.
events: pass parameters to gauge's event functions by const&.
tests: unnecessary forward declarations have been removed from gauge_events test.
events: pass parameters to timer's event functions by const&.
tests: unnecessary forward declarations have been removed from timer_events test.
measuring_points: metric proxies have been added.