Add SAFETY comment on libc::sysconf() call (#11581)

I got an 'undocumented_unsafe_blocks' clippy warning about it. Not sure
why I got the warning now and not before, but in any case a comment is a
good idea.

libs/metrics/src/more_process_metrics.rs

@@ -16,6 +16,7 @@ pub struct Collector {
 const NMETRICS: usize = 2;
 
 static CLK_TCK_F64: Lazy<f64> = Lazy::new(|| {
+    // SAFETY: libc::sysconf is safe, it merely returns a value.
     let long = unsafe { libc::sysconf(libc::_SC_CLK_TCK) };
     if long == -1 {
         panic!("sysconf(_SC_CLK_TCK) failed");