Add TODO comment

Co-authored-by: teor <teor@riseup.net>

zebra-network/src/protocol/external/codec.rs

@@ -174,6 +174,10 @@ impl Codec {
     /// Obtain the size of the body of a given message. This will match the
     /// number of bytes written to the writer provided to `write_body` for the
     /// same message.
+    ///
+    /// TODO: Replace with a size estimate, to avoid multiple serializations
+    /// for large data structures like lists, blocks, and transactions.
+    /// See #1774.
     fn body_length(&self, msg: &Message) -> usize {
         struct FakeWriter(usize);