Add extern declaration and adjust header

src/core/lib/iomgr/threadpool/mpmcqueue.cc

@@ -18,14 +18,16 @@
 
 #include "src/core/lib/iomgr/threadpool/mpmcqueue.h"
 
+#include <grpc/support/port_platform.h>
+
+#include <inttypes.h>
+#include <string.h>
+
 #include <grpc/support/alloc.h>
 #include <grpc/support/cpu.h>
 #include <grpc/support/log.h>
-#include <grpc/support/port_platform.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/time.h>
-#include <inttypes.h>
-#include <string.h>
 
 #include "src/core/lib/debug/stats.h"
 #include "src/core/lib/gprpp/sync.h"

src/core/lib/iomgr/threadpool/mpmcqueue.h

@@ -24,11 +24,14 @@
 #include <grpc/support/alloc.h>
 #include <grpc/support/time.h>
 
+#include "src/core/lib/debug/stats.h"
 #include "src/core/lib/gprpp/atomic.h"
 #include "src/core/lib/gprpp/sync.h"
 
 namespace grpc_core {
 
+extern DebugOnlyTraceFlag thread_pool_trace;
+
 // Abstract base class of a MPMC queue interface
 class MPMCQueueInterface {
  public: