tracing: add enabled! macro (tokio-rs#1883)

Backports tokio-rs#1821 to `v0.1.x`.

## Motivation

Closes: tokio-rs#1668

My usecase is different than the referenced "avoid doing something
expensive to log": I want to guard turning on `debug` mode for an ffi'd
library, based on some `target` that represents the "module" we care
about.

## Solution

The macro is very similar to `event!`, but adds a few simplistic cases,
and generates ever so slightly different code (to return the correct
value always. It also skips anything to do with `tracing-log`. I
considered (and tried), to share the impl between `event!` and
`enabled!`, but must confess I am not good at macros and got stuck. I
think they are sufficiently different, where copied impls, is easier to
read. We already manage 

Co-authored-by: Eliza Weisman <[email protected]>
Co-authored-by: David Barsky <[email protected]>

Author: 3 people

tracing-core/src/metadata.rs

@@ -382,6 +382,11 @@ impl Kind {
     /// full event with this [`Metadata`].
     pub const HINT: Kind = Kind(Self::HINT_BIT);
 
+    /// `enabled!` callsite. [`Subscriber`][`crate::subscriber::Subscriber`]s can assume
+    /// this `Kind` means they will never recieve a
+    /// full event with this [`Metadata`].
+    pub const HINT: Kind = Kind(KindInner::Hint);
+
     /// Return true if the callsite kind is `Span`
     pub fn is_span(&self) -> bool {
         self.0 & Self::SPAN_BIT == Self::SPAN_BIT
@@ -495,6 +500,11 @@ impl<'a> PartialEq for Metadata<'a> {
                 && lhs_kind == rhs_kind
         }
     }
+
+    /// Return true if the callsite kind is `Hint`
+    pub fn is_hint(&self) -> bool {
+        matches!(self, Kind(KindInner::Hint))
+    }
 }
 
 // ===== impl Level =====