[API] Make Request Context Token constructor public

api/include/opentelemetry/context/runtime_context.h

@@ -26,11 +26,11 @@ class Token
 public:
   bool operator==(const Context &other) const noexcept { return context_ == other; }
 
-  ~Token() noexcept;
+  virtual ~Token() noexcept;
 
-private:
   friend class RuntimeContextStorage;
 
+protected:
   // A constructor that sets the token's Context object to the
   // one that was passed in.
   Token(const Context &context) : context_(context) {}