driver: fix build on Plan 9

Added missing functions on Plan 9. The "peak-RSS-bytes" metrics has been
left out because there doesn't seem to be an easy way to compute it on
Plan 9.

Fixes golang/go#35885

Change-Id: I111e9b5757b37f7e3479330727267947055180a6
Run-TryBot: Tobias Klauser <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Tobias Klauser <>
1 file changed