Add per-process schedstat metrics collector

[Luffy-nani]

Feature Request

Currently, the schedstat collector only exports system-wide per-CPU metrics from /proc/schedstat.

However, /proc/[pid]/schedstat exposes per-process scheduler statistics:

• Running time in nanoseconds
• Waiting time in nanoseconds
• Number of timeslices

The procfs library already has ProcSchedstat struct that parses this file, so no new parsing code is needed.

Proposal

Add an opt-in per-process schedstat collector (defaultDisabled, similar to the existing processes collector) that exports:

• node_schedstat_process_running_seconds_total{pid, comm}
• node_schedstat_process_waiting_seconds_total{pid, comm}
• node_schedstat_process_timeslices_total{pid, comm}

Implementation

Would follow the same pattern as collector/processes_linux.go — iterate AllProcs(), call pid.Schedstat() per process, export metrics labeled with pid and comm.