Remove size limit on uexw header

Summary:
Don't truncate error reason in the uexw header.

Meta:

Similar to D92278388 diff, [other parts of `ThriftServer`](https://www.internalfb.com/code/fbsource/[3f91896f158796f40f690d866fab893f29f3b59e]/fbcode/thrift/lib/cpp2/transport/core/ThriftRequest.h?lines=318-319) do not truncate the error reason. This one shouldn't either. remove the max length.

Reviewed By: aristidisp

Differential Revision: D92279288

fbshipit-source-id: 697826dd5fd56e08eb0d499f35e47b2be4d56b31

Author: Sotirios Delimanolis

thrift/lib/cpp2/GeneratedCodeHelper.cpp

@@ -35,6 +35,8 @@ using namespace std;
 using namespace folly;
 using namespace apache::thrift::transport;
 
+THRIFT_FLAG_DEFINE_bool(enforce_uexw_header_length, false);
+
 namespace apache::thrift {
 
 namespace detail {
@@ -295,10 +297,14 @@ void setUserExceptionHeader(
   }
 
   header->setHeader(std::string(detail::kHeaderUex), std::move(exType));
-  header->setHeader(
-      std::string(detail::kHeaderUexw),
-      exReason.size() > kMaxUexwSize ? exReason.substr(0, kMaxUexwSize)
-                                     : std::move(exReason));
+  if (THRIFT_FLAG(enforce_uexw_header_length)) {
+    header->setHeader(
+        std::string(detail::kHeaderUexw),
+        exReason.size() > kMaxUexwSize ? exReason.substr(0, kMaxUexwSize)
+                                       : std::move(exReason));
+  } else {
+    header->setHeader(std::string(detail::kHeaderUexw), std::move(exReason));
+  }
 }
 
 } // namespace