Push glBeginQuery/glEndQuery down into metrics.c

Push glBeginQuery/glEndQuery down into metrics.c

The code in metrics.c was already using dynamic dispatch for OpenGL functions.

But the code in glwrap is now, (and cannot since it relies on "real" OpenGL
header files to ensure the wrapped functions have the correct prototypes).

This resulted in link failures since these functions were not called via
dynamic dispatch.

Gix by adding new metrics_counter_start and metrics_counter_stop functions which
call the glBeginQuery/glEndQuery functions via dynamic dispatch.