metrics: fix build on mips

[name1e5s]

Motivation

Fix build on mips.
close #4469

Solution

Use atomic primitive types provided by crate::loom.

[Darksonn]

Thank you.

[Darksonn]

Can you please disable the metrics feature in loom tests? To do this, modify src/macros/cfg.rs in the following way:

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

[Darksonn]

Thanks.