nfs: allow extra rpc presented
nfs: allow extra rpc presented

Beside RPC calls defined by the RFC, some systems have extra custom RPC
calls for special uses. For example, Synology NAS has 33 RPC calls for
NFSv2

proc2 33 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

For these cases, instead of throwing error, we can just retreive the RPC
stats defined by RPC

Signed-off-by: chiehmin18 <800b30f19dfb65886c5bd61a13a41894e1e6a16a@gmail.com>

prometheus/procfs,prometheus/procfs