feat(agent): migrate RaplCollector from MSR to powercap sysfs

[patryk-przybysz]

Background

Previously, package energy came from MSR registers via /dev/cpu/*/msr, which typically requires root or elevated hardware access. Linux exposes the same counter through powercap sysfs (/sys/class/powercap/intel-rapl:*/energy_uj), which fits a daemon running as an unprivileged user with filesystem read access to those nodes.

Summary

• Replace MSR-based RAPL reads with powercap sysfs energy_uj under intel-rapl package zones.
• Discover package zones by sysfs name (package-N); emit socket 0 only; handle counter wrap via max_energy_range_uj.
• Preserve existing metric semantics: METRIC_TYPE_CPU_POWER_PACKAGE reports energy delta in joules per sample.
• Moves RAPL collection toward a non-root privilege model and further agent daemonization work.

[FW-Nagorko]

Overall a good rewrite, some notes left to be addressed.

[patryk-przybysz]

Good call, thanks for the thorough review.

Addressed the rest: switched discovery to std::filesystem, moved the energy_uj readability check into DiscoverPackages (via ifstream), path is now std::filesystem::path, and package names go through std::regex as you suggested.

Left multi-socket and CPU_POWER_CORES out for now - the MSR path only ever did socket-0 package energy, and cores were never wired up there either. This PR is just the read-path swap. I can open a follow-up issue/PR for the rest if that works.

[FW-Nagorko]

after the changes, LGTM

[patryk-przybysz]

Tested on my machine ✅

Ran into a permission issue at first - the agent needs to be run with sudo (or otherwise granted read access to the powercap sysfs entries) for the metrics to show up. With sudo everything works as expected.

This will be properly resolved once we do proper packaging as a systemd daemon/service - no need to block this PR on that. 😄