Missing support for ProgressRequest in gRPC proxy

[alexandrevilain]

Bug report criteria

• This bug report is not security related, security issues should be disclosed privately via etcd maintainers.
• This is not a support request or question, support requests or questions should be raised in the etcd discussion forums.
• You have read the etcd bug reporting guidelines.
• Existing open issues along with etcd frequently asked questions have been checked and this is not a duplicate.

What happened?

When using k8s 1.32 with ETCD gRPC proxy with the watchList feature gate enabled by default.
We get the following error:

{"level":"error","ts":"2025-04-04T12:13:01.411460Z","caller":"grpcproxy/watch.go:276","msg":"not supported request type by gRPC proxy","request":"progress_request:<> ","stacktrace":"go.etcd.io/etcd/server/v3/proxy/grpcproxy.(*watchProxyStream).recvLoop\n\tgo.etcd.io/etcd/server/v3/proxy/grpcproxy/watch.go:276\ngo.etcd.io/etcd/server/v3/proxy/grpcproxy.(*watchProxy).Watch.func1\n\tgo.etcd.io/etcd/server/v3/proxy/grpcproxy/watch.go:129"}

What did you expect to happen?

ETCD gRPC proxy supporting the pb.WatchRequest_ProgressRequest requests.

How can we reproduce it (as minimally and precisely as possible)?

Starting a k8s APIserver using ETCD gRPC proxy endpoint in the --etcd-servers flags.
The APIserver (if < 1.31) should have the watchList feature gate enabled with --feature-gates=kube:WatchList=true

Anything else we need to know?

We're falling in the default case in the watchProxyStream's recvloop method: https://github.com/etcd-io/etcd/blob/main/server/proxy/grpcproxy/watch.go#L274

Etcd version (please run commands below)

$ etcd --version
etcd Version: 3.5.21
Git SHA: a17edfd
Go Version: go1.23.7
Go OS/Arch: linux/amd64

No "version" command available for gRPC proxy.

Etcd configuration (command line flags or environment variables)

N/A

Etcd debug information (please run commands below, feel free to obfuscate the IP address or FQDN in the output)

N/A

Relevant log output

{"level":"error","ts":"2025-04-04T12:13:00.405820Z","caller":"grpcproxy/watch.go:276","msg":"not supported request type by gRPC proxy","request":"progress_request:<> ","stacktrace":"go.etcd.io/etcd/server/v3/proxy/grpcproxy.(*watchProxyStream).recvLoop\n\tgo.etcd.io/etcd/server/v3/proxy/grpcproxy/watch.go:276\ngo.etcd.io/etcd/server/v3/proxy/grpcproxy.(*watchProxy).Watch.func1\n\tgo.etcd.io/etcd/server/v3/proxy/grpcproxy/watch.go:129"}