Archive pmLookupDesc errors (global derived metrics induced)

[Nathan Scott <nathans@xxxxxxxxxx>]

Hi Mark,

Not sure if you've come across this side-effect, but any archive with
no metrics backing the disk.* derived metrics generates errors now on
use of namespace-expanded metric names.

e.g. pminfo -f -a qa/archives/rattle

This seems like something that's probably going to cause heartache and
confusion for users.  The error messages are fairly cryptic too ...
"disk.dev.await: pmLookupDesc: Metric not defined in the PCP archive log"

We probably need to find a way to suppress these errors for archives?
(since there is no actual error here)

cheers.

--
Nathan