[Backport release-mainnet-1.2.0-rc] fix(process-metrics): build on non-Linux platforms (#4439)

fix(process-metrics): build on non-Linux platforms (#4435)

* fix(process-metrics): build on non-Linux platforms

- Gate procfs dependency to cfg(target_os = "linux") so its build script
  no longer fails on macOS
- Split into a portable ProcessMetrics (memory, cpu_count, uptime via
  sysinfo) and a Linux-only LinuxMetrics extension (/proc, cgroup, PSI)
- Keep the only platform cfg in lib.rs; non-Linux gets an inline no-op ext

* fix(process-metrics): report node_cpu_count via available_parallelism

- System::new().cpus() is empty until an explicit CPU refresh, so the
  gauge reported 0; use std::thread::available_parallelism instead
- Extract cpu_count and process_memory as pure functions with regression
  tests asserting non-zero CPU and memory readings

(cherry picked from commit 809b0a1)

Co-authored-by: Mathis <sveitser@gmail.com>

Author: github-actions[bot]

crates/process-metrics/Cargo.toml

@@ -7,10 +7,12 @@ description = "Process-level Prometheus metrics (RSS, virtual memory, FDs, threa
 
 [dependencies]
 hotshot-types = { workspace = true }
-procfs = { workspace = true }
 sysinfo = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
 
+[target.'cfg(target_os = "linux")'.dependencies]
+procfs = { workspace = true }
+
 [lints]
 workspace = true