pmSortInstances - sort performance metric values on instance identifier
       #include <pcp/pmapi.h>
       void pmSortInstances(pmResult *result)
       cc ... -lpcp
       The routine pmSortInstances may be used  to  guarantee  that  for  each
       performance  metric in the result from pmFetch(3), the instances are in
       ascending instance identifier sequence.
       This is most useful when trying to compute rates from  two  consecutive
       pmFetch(3) results.
       PMAPI(3) and pmFetch(3).
       None.