Add GRPC_CONTEXT_TRAFFIC in grpc_context_index

src/core/lib/channel/context.h

@@ -47,6 +47,9 @@ typedef enum {
   /// Value is a \a census_context.
   GRPC_CONTEXT_TRACING,
 
+  /// Reserved for traffic_class_context.
+  GRPC_CONTEXT_TRAFFIC,
+
   GRPC_CONTEXT_COUNT
 } grpc_context_index;