Add Any protobuf message to CapabilitiesResponse for experimental features (#554)

Author: verios-google

docs/v1/P4Runtime-Spec.adoc

@@ -6032,9 +6032,16 @@ error {
 The `Capabilities` RPC offers a mechanism through which a P4Runtime client can
 discover the capabilities of the P4Runtime server implementation. At the moment,
 the `CapabilitiesRequest` message is empty and the `CapabilitiesResponse`
-message only includes the `p4runtime_api_version` string field. This field must
-be set to the full semantic version string cite:[SemVer] corresponding to the
-version of the P4Runtime API implemented by the server, e.g. "1.1.0-rc.1".
+message includes the `p4runtime_api_version` string field and the `experimental`
+field, which has type `Any` Protobuf message cite:[ProtoAny].
+
+The `p4runtime_api_version` field must be set to the full semantic version string cite:[SemVer]
+corresponding to the version of the P4Runtime API implemented by the server, e.g. "1.1.0-rc.1".
+
+The ` experimental` field is intended to allow parties to experiment with features
+before proposing them for standardization. NOTE: This field will be deprecated after
+`p4runtime.proto` is migrated from `proto3` to proto `editions`. Proto `extensions` cite:[ProtoExtension]
+will be introduced as the standard way to support experimental use cases.
 
 Future versions of P4Runtime may introduce more advanced capability discovery
 features. For example, P4Runtime supports three xref:sec-batch-atomicity[atomicity

docs/v1/resources/theme/references.bib

@@ -100,6 +100,11 @@ @ONLINE { ProtoAny
     url = "https://developers.google.com/protocol-buffers/docs/proto3#any"
 }
 
+@ONLINE { ProtoExtension,
+    title = "Protobuf extension feature",
+    url = "https://protobuf.dev/programming-guides/editions/#extensions"
+}
+
 @ONLINE { gRPCStatus,
     title = "the gRPC $Status$ class",
     url = "https://github.com/grpc/grpc/blob/master/include/grpcpp/impl/codegen/status.h"

go/p4/v1/p4runtime.pb.go

@@ -840,4 +840,8 @@ message CapabilitiesResponse {
   // The full semantic version string (e.g. "1.1.0-rc.1") corresponding to the
   // version of the P4Runtime API currently implemented by the server.
   string p4runtime_api_version = 1;
+
+  // Used for experimental features before proposing them for standardization.
+  // Added in 1.5.0.
+  .google.protobuf.Any experimental = 999;
 }