Prepare o2dpg_sim_metrics.py for the new workflow runner

[sawenzel]

This is the first step of a refactoring of MC/bin/o2_dpg_workflow_runner.py into a modular package, presented at CHEP 2026. It updates the metrics tool to read what that runner writes, and fixes two numbers it was already getting wrong.

• cgroup_cpu and cgroup_mem are read when the runner ran under --systemd-run, and slice-level CPU is preferred over the sum of per-task psutil readings.
• cgroup_global and global_init_task rows are internal markers and no longer enter the per-task statistics.
• lifetime was an iteration count and is now seconds, which is what the new schedule simulator reads it as. update_resource_estimates() only reads pss.max and cpu.mean, so the current runner is unaffected.
• json-stat reads the *.log_time files written by GNU time, and falls back to user time where a task was too short for its 10 ms wall resolution.
• The runtime estimate assumed every monitor iteration takes 5 seconds. On a 12.2 s run it reported 65 s. Walltime now comes from the log timestamps, which gives 10.9 s for the same run.
• min_lifetime was computed with .max(). It now uses .min(), and a std is reported alongside.

https://indico.cern.ch/event/1471803/contributions/6967072/

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