The encoding of a performance metric's dimensionality and scale uses a pmUnits structure; see
pmLookupDesc(3).
As an aid to labeling graphs and tables, or for error messages, pmUnitsStr will take a dimension and
scale specification as per pu, and return the corresponding text string. The pmUnitsStr_r function does
the same, but stores the result in a user-supplied buffer buf of length buflen, which should have room
for at least 60 bytes. If buflen is less than 60 then pmUnitsStr_r returns NULL and buf is not changed.
For example {1,-2,0,PM_SPACE_MBYTE,PM_TIME_SEC,0}, as the value of *pu gives the result string Mbyte/sec^2.
The string value result from pmUnitsStr is held in a single static buffer, so the returned value is only
valid until the next call to pmUnitsStr.
If the ``count'' dimension is non-zero, and the ``count'' scale is not zero, then the text string will
include a decimal scaling factor, e.g. countx10^6.
As a special case, if all components of the dimension are zero, then the ``count'' scale is used to
produce the text. If this scale is zero the result is an empty string, otherwise the result is of the
form x10^2.