subscriber: use macros for module declarations (#1009)

## Motivation

This is code refactoring. Currently, there is a lot of code that declares
modules as follows:

```rust
#[cfg(feature = "registry")]
#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
pub use registry::Registry;

#[cfg(feature = "registry")]
#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
pub fn registry() -> Registry {
    Registry::default()
}
```

It is verbose to write a module declaration multiple times using the feature
attribute. Also, if the definition for the same feature spans multiple places,
it will be a little harder to read the code.

## Solution

You can combine features attributes in one place by writing as follows. It also
eliminates the need to write the same code over and over again.

```rust
cfg_feature!("registry", {
    pub use registry::Registry;

    pub fn registry() -> Registry {
        Registry::default()
    }
});

```

If this code is accepted, we will extend the declaration of the module using
macros to other files as well as `tracing-subscriber/lib.rs`.

tracing-subscriber/src/lib.rs

@@ -98,26 +98,10 @@
 use tracing_core::span::Id;
 
 #[macro_use]
-macro_rules! try_lock {
-    ($lock:expr) => {
-        try_lock!($lock, else return)
-    };
-    ($lock:expr, else $els:expr) => {
-        if let Ok(l) = $lock {
-            l
-        } else if std::thread::panicking() {
-            $els
-        } else {
-            panic!("lock poisoned")
-        }
-    };
-}
+mod macros;
 
 pub mod field;
 pub mod filter;
-#[cfg(feature = "fmt")]
-#[cfg_attr(docsrs, doc(cfg(feature = "fmt")))]
-pub mod fmt;
 pub mod layer;
 pub mod prelude;
 pub mod registry;
@@ -132,24 +116,20 @@ pub use filter::EnvFilter;
 
 pub use layer::Layer;
 
-#[cfg(feature = "registry")]
-#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
-pub use registry::Registry;
-
-///
-#[cfg(feature = "registry")]
-#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
-pub fn registry() -> Registry {
-    Registry::default()
-}
+cfg_feature!("fmt", {
+    pub mod fmt;
+    pub use fmt::fmt;
+    pub use fmt::Subscriber as FmtSubscriber;
+});
 
-#[cfg(feature = "fmt")]
-#[cfg_attr(docsrs, doc(cfg(feature = "fmt")))]
-pub use fmt::Subscriber as FmtSubscriber;
+cfg_feature!("registry", {
+    pub use registry::Registry;
 
-#[cfg(feature = "fmt")]
-#[cfg_attr(docsrs, doc(cfg(feature = "fmt")))]
-pub use fmt::fmt;
+    ///
+    pub fn registry() -> Registry {
+        Registry::default()
+    }
+});
 
 use std::default::Default;
 /// Tracks the currently executing span on a per-thread basis.

tracing-subscriber/src/macros.rs

@@ -0,0 +1,24 @@
+macro_rules! try_lock {
+    ($lock:expr) => {
+        try_lock!($lock, else return)
+    };
+    ($lock:expr, else $els:expr) => {
+        if let Ok(l) = $lock {
+            l
+        } else if std::thread::panicking() {
+            $els
+        } else {
+            panic!("lock poisoned")
+        }
+    };
+}
+
+macro_rules! cfg_feature {
+    ($name:literal, { $($item:item)* }) => {
+        $(
+            #[cfg(feature = $name)]
+            #[cfg_attr(docsrs, doc(cfg(feature = $name)))]
+            $item
+        )*
+    }
+}