Add a hook for the default constructor of ClientContext.

include/grpc++/client_context.h

@@ -280,6 +280,16 @@ class ClientContext {
   /// There is no guarantee the call will be cancelled.
   void TryCancel();
 
+  /// Global Callbacks
+  ///
+  /// Can be set exactly once per application to install hooks whenever
+  /// a client context is constructed.
+  class GlobalCallbacks {
+   public:
+    virtual void DefaultConstructor(ClientContext* context) = 0;
+  };
+  static void SetGlobalCallbacks(GlobalCallbacks* callbacks);
+
  private:
   // Disallow copy and assign.
   ClientContext(const ClientContext&);

src/cpp/client/client_context.cc

@@ -45,12 +45,22 @@
 
 namespace grpc {
 
+class DefaultGlobalCallbacks GRPC_FINAL : public ClientContext::GlobalCallbacks {
+ public:
+  void DefaultConstructor(ClientContext* context) GRPC_OVERRIDE {}
+};
+
+static DefaultGlobalCallbacks g_default_callbacks;
+static ClientContext::GlobalCallbacks* g_callbacks = &g_default_callbacks;
+
 ClientContext::ClientContext()
     : initial_metadata_received_(false),
       call_(nullptr),
       call_canceled_(false),
       deadline_(gpr_inf_future(GPR_CLOCK_REALTIME)),
-      propagate_from_call_(nullptr) {}
+      propagate_from_call_(nullptr) {
+  g_callbacks->DefaultConstructor(this);
+}
 
 ClientContext::~ClientContext() {
   if (call_) {
@@ -124,4 +134,11 @@ grpc::string ClientContext::peer() const {
   return peer;
 }
 
+void ClientContext::SetGlobalCallbacks(GlobalCallbacks* callbacks) {
+  GPR_ASSERT(g_callbacks == &g_default_callbacks);
+  GPR_ASSERT(callbacks != NULL);
+  GPR_ASSERT(callbacks != &g_default_callbacks);
+  g_callbacks = callbacks;
+}
+
 }  // namespace grpc