Logging macro `if_log_enabled` checks both `log` and `always-log` crate
features to select which macro version to use. Unfortunately, `log` feature
cannot be set by crate consumers in either existing form or if added
to `Cargo.toml`, probably because of a name clash with `log` crate:

```diff
$ git diff Cargo.toml
diff --git a/tracing/Cargo.toml b/tracing/Cargo.toml
index 37bb61f..e030d27 100644
--- a/tracing/Cargo.toml
+++ b/tracing/Cargo.toml
@@ -63,7 +63,7 @@ release_max_level_trace = []
 async-await = []

 std = ["tracing-core/std"]
+log = ["log"]
 log-always = ["log"]
 attributes = ["tracing-attributes"]
 ```

```
Caused by:
  optional dependency `log` is not included in any feature
  Make sure that `dep:log` is included in one of features in the [features] table.
```

I'm renaming `log` feature to `logging` to work around that.

There's a slight improvement in throughput with logging disabled:

```
global/collector/span_no_fields
                        time:   [7.9682 ns 7.9704 ns 7.9729 ns]
                        change: [-13.902% -13.812% -13.720%] (p = 0.00 < 0.05)
                        Performance has improved.
```