Rename grpc.Params.headers to metadata?

[na--]

Prompted by https://community.k6.io/t/grpc-connect-without-port-no/2326/4

We currently allow custom metadata to be configured with the headers parameter:

k6/js/modules/k6/grpc/client.go

Lines 416 to 428 in 434b2e7

	case "headers":
	rawHeaders, ok := v.(map[string]interface{})
	if !ok {
	return nil, errors.New("headers must be an object with key-value pairs")
	}
	for hk, kv := range rawHeaders {
	// TODO(rogchap): Should we manage a string slice?
	strVal, ok := kv.(string)
	if !ok {
	return nil, fmt.Errorf("header %q value must be a string", hk)
	}
	ctx = metadata.AppendToOutgoingContext(ctx, hk, strVal)
	}

But even though that might reflect the gRPC implementation details, its proper terminology and parameter name should probably be metadata:

• https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md
• https://pkg.go.dev/google.golang.org/grpc/metadata
• https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md