Fix parca agent off-cpu profile type

[metalmatze]

Currently, the profile type for Off-CPU is wrong.

Compare these two:
On-CPU: parca_agent:samples:count:cpu:nanoseconds:delta
Off-CPU: parca_agent:wallclock:nanoseconds:samples:count:delta

We need to make this change:

- parca_agent:wallclock:nanoseconds:samples:count:delta
+ parca_agent:samples:count:wallclock:nanoseconds:delta

The Parca UI should then be able to extract the correct unit from the profile type.
Maybe we'll have to add wallclock as an additional option somewhere.