[API] Change the param-pack unpacking order to start from left to right (open-telemetry#3296)

* Change the param-pack unpacking order to start from left to right

* Add fallback for c++14 and below

Author: ThomsonTan

api/include/opentelemetry/logs/logger.h

@@ -72,8 +72,23 @@ class Logger
       return;
     }
 
-    IgnoreTraitResult(detail::LogRecordSetterTrait<typename std::decay<ArgumentType>::type>::Set(
-        log_record.get(), std::forward<ArgumentType>(args))...);
+    //
+    // Keep the parameter pack unpacking order from left to right because left
+    // ones are usually more important like severity and event_id than the
+    // attributes. The left to right unpack order could pass the more important
+    // data to processors to avoid caching and memory allocating.
+    //
+#if __cplusplus <= 201402L
+    // C++14 does not support fold expressions for parameter pack expansion.
+    int dummy[] = {(detail::LogRecordSetterTrait<typename std::decay<ArgumentType>::type>::Set(
+                        log_record.get(), std::forward<ArgumentType>(args)),
+                    0)...};
+    IgnoreTraitResult(dummy);
+#else
+    IgnoreTraitResult((detail::LogRecordSetterTrait<typename std::decay<ArgumentType>::type>::Set(
+                           log_record.get(), std::forward<ArgumentType>(args)),
+                       ...));
+#endif
 
     EmitLogRecord(std::move(log_record));
   }