Support restricting exposed enum values in OpenAPI for specific field usages

Feature

Problem

When using protoc-gen-openapiv2, enum fields always expose the full enum definition in generated Swagger/OpenAPI schemas.

Example:

enum ResourceType { RESOURCE_TYPE_UNSPECIFIED = 0; RESOURCE_TYPE_USER = 1; RESOURCE_TYPE_GROUP = 2; RESOURCE_TYPE_SERVICE = 3; RESOURCE_TYPE_INTERNAL = 4; }

message ResourceRequest { ResourceType resource_type = 1; }

The generated OpenAPI schema exposes all enum values globally.

However, in some APIs we want:

strong typing internally using protobuf enums generated language constants enum validation

but only a restricted subset (or alternate representation) exposed in OpenAPI for a particular field usage.

For example, RESOURCE_TYPE_INTERNAL may be intended only for internal services and should not appear in public API documentation.

Currently the only workaround is to:

change enum fields to string duplicate enum names manually in validation annotations maintain synchronization manually between enum definitions and string validation lists

Example workaround:

string resource_type = 1 [ (validate.rules).string = { in: [ "RESOURCE_TYPE_USER", "RESOURCE_TYPE_GROUP", "RESOURCE_TYPE_SERVICE" ] } ];

This introduces:

duplicated source of truth maintenance burden risk of enum/string drift loss of protobuf enum typing in API contracts

Requested Feature

Support field-level OpenAPI enum projection/restriction.

Example idea:

ResourceType resource_type = 1 [ (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = { enum_subset: [ "RESOURCE_TYPE_USER", "RESOURCE_TYPE_GROUP", "RESOURCE_TYPE_SERVICE" ] } ];

Expected behavior:

protobuf field remains enum typed generated SDKs retain enum safety OpenAPI schema exposes only selected enum values for that field avoids converting enums to strings purely for documentation concerns Additional Context

This is especially useful for:

public vs internal API exposure partial enum availability