Revert ordering of members

Author: chusitoo

api/include/opentelemetry/context/context.h

@@ -94,10 +94,10 @@ class Context
   {
     char *key_ = nullptr;
 
-    size_t key_length_ = 0UL;
-
     nostd::shared_ptr<DataList> next_;
 
+    size_t key_length_ = 0UL;
+
     ContextValue value_;
 
     DataList() : next_{nullptr} {}