metrics: stabilize RuntimeMetrics::worker_count

This PR stabilizes a single metric API to start the process of stabilizing metrics.
Future work will continue to stabilize more metrics.

This PR also introduces a `metrics` feature.

Refs: tokio-rs#6546

tokio/Cargo.toml

@@ -31,6 +31,7 @@ full = [
   "io-util",
   "io-std",
   "macros",
+  "metrics",
   "net",
   "parking_lot",
   "process",
@@ -46,6 +47,7 @@ io-util = ["bytes"]
 # stdin, stdout, stderr
 io-std = []
 macros = ["tokio-macros"]
+metrics = []
 net = [
   "libc",
   "mio/os-poll",

tokio/src/lib.rs

@@ -471,6 +471,7 @@ compile_error!("The `tokio_taskdump` feature requires `--cfg tokio_unstable`.");
 
 #[cfg(all(
     tokio_taskdump,
+    not(doc),
     not(all(
         target_os = "linux",
         any(target_arch = "aarch64", target_arch = "x86", target_arch = "x86_64")

tokio/src/macros/cfg.rs

@@ -215,6 +215,17 @@ macro_rules! cfg_macros {
     }
 }
 
+macro_rules! cfg_stable_metrics {
+    ($($item:item)*) => {
+        $(
+            #[cfg(not(loom))]
+            #[cfg(feature = "metrics")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "metrics")))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_metrics {
     ($($item:item)*) => {
         $(

tokio/src/runtime/handle.rs

@@ -395,15 +395,15 @@ impl Handle {
     }
 }
 
-cfg_metrics! {
+cfg_rt! {
     use crate::runtime::RuntimeMetrics;
+}
 
-    impl Handle {
-        /// Returns a view that lets you get information about how the runtime
-        /// is performing.
-        pub fn metrics(&self) -> RuntimeMetrics {
-            RuntimeMetrics::new(self.clone())
-        }
+impl Handle {
+    /// Returns a view that lets you get information about how the runtime
+    /// is performing.
+    pub fn metrics(&self) -> RuntimeMetrics {
+        RuntimeMetrics::new(self.clone())
     }
 }
 

tokio/src/runtime/metrics/mod.rs

@@ -8,6 +8,9 @@
 //! [unstable]: crate#unstable-features
 #![allow(clippy::module_inception)]
 
+mod runtime;
+pub use runtime::RuntimeMetrics;
+
 cfg_metrics! {
     mod batch;
     pub(crate) use batch::MetricsBatch;
@@ -17,10 +20,6 @@ cfg_metrics! {
     #[allow(unreachable_pub)] // rust-lang/rust#57411
     pub use histogram::HistogramScale;
 
-    mod runtime;
-    #[allow(unreachable_pub)] // rust-lang/rust#57411
-    pub use runtime::RuntimeMetrics;
-
     mod scheduler;
     pub(crate) use scheduler::SchedulerMetrics;