Add a destructor callback as well

include/grpc++/client_context.h

@@ -283,10 +283,11 @@ class ClientContext {
   /// Global Callbacks
   ///
   /// Can be set exactly once per application to install hooks whenever
-  /// a client context is constructed.
+  /// a client context is constructed and destructed.
   class GlobalCallbacks {
    public:
     virtual void DefaultConstructor(ClientContext* context) = 0;
+    virtual void Destructor(ClientContext* context) = 0;
   };
   static void SetGlobalCallbacks(GlobalCallbacks* callbacks);
 

src/cpp/client/client_context.cc

@@ -48,6 +48,7 @@ namespace grpc {
 class DefaultGlobalCallbacks GRPC_FINAL : public ClientContext::GlobalCallbacks {
  public:
   void DefaultConstructor(ClientContext* context) GRPC_OVERRIDE {}
+  void Destructor(ClientContext* context) GRPC_OVERRIDE {}
 };
 
 static DefaultGlobalCallbacks g_default_callbacks;
@@ -66,6 +67,7 @@ ClientContext::~ClientContext() {
   if (call_) {
     grpc_call_destroy(call_);
   }
+  g_callbacks->Destructor(this);
 }
 
 std::unique_ptr<ClientContext> ClientContext::FromServerContext(