suggestion
func ratelimitToMetadata(req *pb.RateLimitRequest) *structpb.Struct {
    fields := make(map[string]*structpb.Value)
    
    // Domain
    fields["domain"] = structpb.NewStringValue(req.Domain)
    
    // Descriptors
    descriptorsValues := make([]*structpb.Value, 0, len(req.Descriptors))
    for _, descriptor := range req.Descriptors {
        s := descriptorToStruct(descriptor)
        if s == nil {
            continue
        }
        descriptorsValues = append(descriptorsValues, structpb.NewStructValue(s))
    }
    fields["descriptors"] = structpb.NewListValue(&structpb.ListValue{
        Values: descriptorsValues,
    })
    
    // HitsAddend
    if hitsAddend := req.GetHitsAddend(); hitsAddend != 0 {
        fields["hitsAddend"] = structpb.NewNumberValue(float64(hitsAddend))
    }
    
    return &structpb.Struct{Fields: fields}
}

suggestion
func descriptorToStruct(descriptor *ratelimitv3.RateLimitDescriptor) *structpb.Struct {
    if descriptor == nil {
        return nil
    }
    
    fields := make(map[string]*structpb.Value)
    
    // Entries
    entriesValues := make([]*structpb.Value, 0, len(descriptor.Entries))
    for _, entry := range descriptor.Entries {
        val := fmt.Sprintf("%s=%s", entry.GetKey(), entry.GetValue())
        entriesValues = append(entriesValues, structpb.NewStringValue(val))
    }
    fields["entries"] = structpb.NewListValue(&structpb.ListValue{
        Values: entriesValues,
    })
    
    // Limit
    if descriptor.GetLimit() != nil {
        fields["limit"] = structpb.NewStringValue(descriptor.Limit.String())
    }
    
    // HitsAddend
    if hitsAddend := descriptor.GetHitsAddend(); hitsAddend != nil {
        fields["hitsAddend"] = structpb.NewNumberValue(float64(hitsAddend.GetValue()))
    }
    
    return &structpb.Struct{Fields: fields}
}