[coap] block2 fix PrepareNextBlockRequest to not return error (#12098)

src/core/coap/coap.cpp

@@ -636,10 +636,14 @@ Error CoapBase::PrepareNextBlockRequest(Message::BlockType aType,
     bool             isOptionSet = false;
     uint16_t         blockOption = 0;
     Option::Iterator iterator;
+    Metadata         metadata;
 
     blockOption = (aType == Message::kBlockType1) ? kOptionBlock1 : kOptionBlock2;
 
     aRequest.Init(kTypeConfirmable, static_cast<ot::Coap::Code>(aRequestOld.GetCode()));
+    // Iterate after metadata copied and removed.
+    metadata.ReadFrom(aRequestOld);
+    metadata.RemoveFrom(aRequestOld);
     // Per RFC 7959, all requests in a block-wise transfer MUST use the same token.
     IgnoreError(aRequest.SetTokenFromMessage(aRequestOld));
     SuccessOrExit(error = iterator.Init(aRequestOld));
@@ -686,6 +690,8 @@ Error CoapBase::PrepareNextBlockRequest(Message::BlockType aType,
         aRequest.SetMoreBlocksFlag(aMoreBlocks);
     }
 
+    error = metadata.AppendTo(aRequestOld);
+
 exit:
     return error;
 }