tests(helpers): add EchoHeaders grpc rpc

This will improve testability and reliability of some gRPC tests.

spec/fixtures/grpc/proto/targetservice.proto

@@ -100,4 +100,13 @@ message EchoMsg {
   string nullable = 2;
 }
 
+message Header {
+  string key = 1;
+  string value = 2;
+}
+
+message Headers {
+  repeated Header headers = 1;
+}
+
 message Void {}

spec/fixtures/grpc/target.go

@@ -6,11 +6,15 @@ import (
 	"log"
 	"net"
 	"time"
+	"strings"
 
 	pb "grpc/targetservice"
 
 	"google.golang.org/grpc"
 	"google.golang.org/protobuf/types/known/timestamppb"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 const (
@@ -53,6 +57,22 @@ func (s *server) Echo(ctx context.Context, in *pb.EchoMsg) (*pb.EchoMsg, error)
 	return in, nil
 }
 
+func (s *server) EchoHeaders(ctx context.Context, in *pb.Void) (*pb.Headers, error) {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		return nil, status.Errorf(codes.DataLoss, "UnaryEcho: failed to get metadata")
+	}
+	headers := &pb.Headers{}
+	for k, v := range md {
+		header := &pb.Header{
+			Key: k,
+			Value: strings.Join(v, ", "),
+		}
+		headers.Headers = append(headers.Headers, header)
+	}
+	return headers, nil
+}
+
 func main() {
 	lis, err := net.Listen("tcp", port)
 	if err != nil {