metrics: fix build on mips (#4475)

tokio/src/macros/cfg.rs

@@ -177,9 +177,9 @@ macro_rules! cfg_macros {
 macro_rules! cfg_metrics {
     ($($item:item)*) => {
         $(
-            // For now, metrics is always enabled. When stabilized, it might
-            // have a dedicated feature flag.
-            #[cfg(tokio_unstable)]
+            // For now, metrics is only disabled in loom tests.
+            // When stabilized, it might have a dedicated feature flag.
+            #[cfg(all(tokio_unstable, not(loom)))]
             #[cfg_attr(docsrs, doc(cfg(tokio_unstable)))]
             $item
         )*
@@ -189,7 +189,7 @@ macro_rules! cfg_metrics {
 macro_rules! cfg_not_metrics {
     ($($item:item)*) => {
         $(
-            #[cfg(not(tokio_unstable))]
+            #[cfg(not(all(tokio_unstable, not(loom))))]
             $item
         )*
     }

tokio/src/runtime/metrics/worker.rs

@@ -1,5 +1,5 @@
-use std::sync::atomic::Ordering::Relaxed;
-use std::sync::atomic::{AtomicU64, AtomicUsize};
+use crate::loom::sync::atomic::Ordering::Relaxed;
+use crate::loom::sync::atomic::{AtomicU64, AtomicUsize};
 
 /// Retreive runtime worker metrics.
 ///