Support binary metadata

Add support for binary metadata (postfixed with `-bin`).

This ports changes from the grafana/k6#3234

grpc/client_test.go

@@ -335,6 +335,31 @@ func TestClient(t *testing.T) {
 				}
 			`},
 		},
+		{
+			name: "RequestBinHeaders",
+			initString: codeBlock{
+				code: `
+				var client = new grpc.Client();
+				client.load([], "../grpc/testdata/grpc_testing/test.proto");`,
+			},
+			setup: func(tb *httpmultibin.HTTPMultiBin) {
+				tb.GRPCStub.EmptyCallFunc = func(ctx context.Context, _ *grpc_testing.Empty) (*grpc_testing.Empty, error) {
+					md, ok := metadata.FromIncomingContext(ctx)
+					if !ok || len(md["x-load-tester-bin"]) == 0 || md["x-load-tester-bin"][0] != string([]byte{2, 200}) {
+						return nil, status.Error(codes.FailedPrecondition, "")
+					}
+
+					return &grpc_testing.Empty{}, nil
+				}
+			},
+			vuString: codeBlock{code: `
+				client.connect("GRPCBIN_ADDR");
+				var resp = client.invoke("grpc.testing.TestService/EmptyCall", {}, { metadata: { "X-Load-Tester-bin": new Uint8Array([2, 200]) } })
+				if (resp.status !== grpc.StatusOK) {
+					throw new Error("failed to send correct headers in the request")
+				}
+			`},
+		},
 		{
 			name: "ResponseMessage",
 			initString: codeBlock{

grpc/params.go

@@ -47,12 +47,23 @@ func newCallParams(vu modules.VU, input goja.Value) (*callParams, error) {
 				return result, errors.New("metadata must be an object with key-value pairs")
 			}
 			for hk, kv := range rawHeaders {
-				strval, ok := kv.(string)
-				if !ok {
+				var val string
+
+				// The gRPC spec defines that Binary-valued keys end in -bin
+				// https://grpc.io/docs/what-is-grpc/core-concepts/#metadata
+				if strings.HasSuffix(hk, "-bin") {
+					var binVal []byte
+					if binVal, ok = kv.([]byte); !ok {
+						return result, fmt.Errorf("metadata %q value must be binary", hk)
+					}
+
+					// https://github.com/grpc/grpc-go/blob/v1.57.0/Documentation/grpc-metadata.md#storing-binary-data-in-metadata
+					val = string(binVal)
+				} else if val, ok = kv.(string); !ok {
 					return result, fmt.Errorf("metadata %q value must be a string", hk)
 				}
 
-				result.Metadata.Append(hk, strval)
+				result.Metadata.Append(hk, val)
 			}
 		case "tags":
 			if err := common.ApplyCustomUserTags(rt, &result.TagsAndMeta, params.Get(k)); err != nil {