[protobuf] JS bundle bloat due to unoptimizable proto extensions

Hi, I was checking into Messages for Web's bundle size again today and noticed we still have large amounts of bloat coming from binary serialization code retained due to protobuf extensions (as described in b/146146114). I noticed this issue hasn't been assigned to anyone for over half a year.

[protobuf] Divergent implementations of JSON transcoding/control of default value behaviour

Hello everyone, In our setup we use Google Cloud Enpoints and the esp container in our cloud environments, but grpc-gateway for our local development. We see a difference in behaviour when transcoding protobuf to JSON, namely in how protobufs default values are transcoded. This has lead to some