subscriber: fix unused method warnings for Extensions (#1686)

## Motivation

Currently, there are some `pub(crate)` methods on the `Extensions`,
`ExtensionsMut`, and `ExtensionsInner` types that are only used when the
"registry" feature is enabled. However, the _types_ exist when the "std"
feature flag is enabled, so those methods produce unused method
warnings.

## Solution

This commit adds `#[cfg(feature = "registry")]` to those methods, fixing
the warning.

tracing-subscriber/src/registry/extensions.rs

@@ -41,6 +41,7 @@ pub struct Extensions<'a> {
 }
 
 impl<'a> Extensions<'a> {
+ #[cfg(feature = "registry")]
  pub(crate) fn new(inner: RwLockReadGuard<'a, ExtensionsInner>) -> Self {
  Self { inner }
  }
@@ -59,6 +60,7 @@ pub struct ExtensionsMut<'a> {
 }
 
 impl<'a> ExtensionsMut<'a> {
+ #[cfg(feature = "registry")]
  pub(crate) fn new(inner: RwLockWriteGuard<'a, ExtensionsInner>) -> Self {
  Self { inner }
  }
@@ -120,6 +122,7 @@ pub(crate) struct ExtensionsInner {
 
 impl ExtensionsInner {
  /// Create an empty `Extensions`.
+ #[cfg(any(test, feature = "registry"))]
  #[inline]
  pub(crate) fn new() -> ExtensionsInner {
  ExtensionsInner {
@@ -179,6 +182,7 @@ impl ExtensionsInner {
  ///
  /// This permits the hash map allocation to be pooled by the registry so
  /// that future spans will not need to allocate new hashmaps.
+ #[cfg(any(test, feature = "registry"))]
  pub(crate) fn clear(&mut self) {
  self.map.clear();
  }