A example of a UnaryServerInterceptor

ServerCount - UnaryInterceptor.go

// PingCounter is a struct that keeps track of how many Pings that are performed
type PingCounter struct {
	Pings int
}

// ServerCount is a gRPC UnaryServerInterceptor that will count number of API calls.
func (pc *PingCounter) ServerCount(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (response interface{}, err error) {
	// Append to PingCounts
	pc.Pings++
	// We want to extract metadata from the incomming context.
	// We dont create a new context since we dont wanna overwrite old metadata
	meta, ok := metadata.FromIncomingContext(ctx)
	if !ok {
		return nil, errors.New("could not grab metadata from context")
	}
	// Set ping-counts into the current ping value
	meta.Set("ping-counts", fmt.Sprintf("%d", pc.Pings))
	// Metadata is sent on its own, so we need to send the header. There is also something called Trailer
	grpc.SendHeader(ctx, meta)
	// Last but super important, execute the handler so that the actualy gRPC request is also performed
	return handler(ctx, req)
}