Allow linking rustc and rustdoc against the same single tracing crate

By consecutively initializing `tracing` and `rustc_log`, Rustdoc assumes
that these involve 2 different tracing crates.

I would like to be able to build rustdoc against the same tracing crate
that rustc_log is also built against. Previously this arrangement would
crash rustdoc:

    thread 'main' panicked at rust/compiler/rustc_log/src/lib.rs:142:65:
    called `Result::unwrap()` on an `Err` value: SetGlobalDefaultError("a global default trace dispatcher has already been set")
    stack backtrace:
       0: rust_begin_unwind
       1: core::panicking::panic_fmt
       2: core::result::unwrap_failed
       3: rustc_log::init_logger
       4: rustc_driver_impl::init_logger
       5: rustdoc::main
    note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

    error: the compiler unexpectedly panicked. this is a bug.

    note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-rustdoc&template=ice.md

    note: please make sure that you have updated to the latest nightly

    query stack during panic:
    end of query stack

compiler/rustc_log/src/lib.rs

@@ -37,6 +37,7 @@ use std::env::{self, VarError};
 use std::fmt::{self, Display};
 use std::io::{self, IsTerminal};
 
+use tracing::dispatcher::SetGlobalDefaultError;
 use tracing_core::{Event, Subscriber};
 use tracing_subscriber::filter::{Directive, EnvFilter, LevelFilter};
 use tracing_subscriber::fmt::FmtContext;
@@ -131,10 +132,10 @@ pub fn init_logger(cfg: LoggerConfig) -> Result<(), Error> {
                 .without_time()
                 .event_format(BacktraceFormatter { backtrace_target });
             let subscriber = subscriber.with(fmt_layer);
-            tracing::subscriber::set_global_default(subscriber).unwrap();
+            tracing::subscriber::set_global_default(subscriber)?;
         }
         Err(_) => {
-            tracing::subscriber::set_global_default(subscriber).unwrap();
+            tracing::subscriber::set_global_default(subscriber)?;
         }
     };
 
@@ -180,6 +181,7 @@ pub enum Error {
     InvalidColorValue(String),
     NonUnicodeColorValue,
     InvalidWraptree(String),
+    AlreadyInit(SetGlobalDefaultError),
 }
 
 impl std::error::Error for Error {}
@@ -199,6 +201,13 @@ impl Display for Error {
                 formatter,
                 "invalid log WRAPTREE value '{value}': expected a non-negative integer",
             ),
+            Error::AlreadyInit(tracing_error) => Display::fmt(tracing_error, formatter),
         }
     }
 }
+
+impl From<SetGlobalDefaultError> for Error {
+    fn from(tracing_error: SetGlobalDefaultError) -> Self {
+        Error::AlreadyInit(tracing_error)
+    }
+}