Begin re-factoring metrics.c into separate context.c and metrics-info.c

The code in metrics.c was getting a bit unwieldy. Some of it is pushed
up into the existing context.c file. Other portions (specifically, the
code which queries the names of all available performance monitors) is
pushed down into a new metrics-info.c.

There's not yet very hard boundaries between these files yet, (they
are all sharing their internals in header files), but this gives some
structure for future cleanups.