Pass peer string to ChannelData ctor when available.

include/grpc++/channel_filter.h

@@ -62,8 +62,13 @@ class ChannelData {
                                 grpc_channel_element *elem,
                                 grpc_transport_op *op);
 
+  const char* peer() const { return peer_; }
+
  protected:
-  explicit ChannelData(const grpc_channel_args &) {}
+  ChannelData(const grpc_channel_args &args, const char *peer) : peer_(peer) {}
+
+ private:
+  const char *peer_;  // Do not own.
 };
 
 // Represents call data.
@@ -96,8 +101,12 @@ class ChannelFilter {
   static void InitChannelElement(grpc_exec_ctx *exec_ctx,
                                  grpc_channel_element *elem,
                                  grpc_channel_element_args *args) {
+    const char* peer = args->optional_transport
+                       ? grpc_transport_get_peer(exec_ctx,
+                                                 args->optional_transport)
+                       : nullptr;
     // Construct the object in the already-allocated memory.
-    new (elem->channel_data) ChannelDataType(*args->channel_args);
+    new (elem->channel_data) ChannelDataType(*args->channel_args, peer);
   }
 
   static void DestroyChannelElement(grpc_exec_ctx *exec_ctx,

src/core/lib/transport/transport.h

@@ -43,6 +43,10 @@
 #include "src/core/lib/transport/byte_stream.h"
 #include "src/core/lib/transport/metadata_batch.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* forward declarations */
 typedef struct grpc_transport grpc_transport;
 
@@ -264,4 +268,8 @@ void grpc_transport_destroy(grpc_exec_ctx *exec_ctx, grpc_transport *transport);
 char *grpc_transport_get_peer(grpc_exec_ctx *exec_ctx,
                               grpc_transport *transport);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* GRPC_CORE_LIB_TRANSPORT_TRANSPORT_H */