I'm trying to implement an interceptor for logging in gRPC servers and clients.
In the interceptor I'd like to log all the headers and trailers.
I tried using the following code to read all the key-values from the Metadata and convert them to a string representation to print them to the screen:

    private String metadataToString(final Metadata metadata) {
        if (metadata == null) {
            return "";
        }

        var sb = new StringBuilder();

        for (String key : metadata.keys())
        {
            var metadataKey = Metadata.Key.of(key, Metadata.ASCII_STRING_MARSHALLER);

            sb.append(
                String.format(
                    "%s: %s\n",
                    key,
                    metadata.get(metadataKey)));
        }

        return sb.toString();
    }

I tested it in a client interceptor to log the headers and the trailers. This works fine if the gRPC request is successful.
But if the request fails, then apparently the framework automatically adds a trailer with the key :status. And in that case, the code above fails with the following exception:

java.lang.IllegalArgumentException: Invalid character ':' in key name ':status'
        at com.googlemon.base.Preconditions.checkArgument(Preconditions.java:273)
        at io.grpc.Metadata$Key.validateName(Metadata.java:754)
        at io.grpc.Metadata$Key.<init>(Metadata.java:762)
        at io.grpc.Metadata$Key.<init>(Metadata.java:671)
        at io.grpc.Metadata$AsciiKey.<init>(Metadata.java:971)
        at io.grpc.Metadata$AsciiKey.<init>(Metadata.java:966)
        at io.grpc.Metadata$Key.of(Metadata.java:708)
        at io.grpc.Metadata$Key.of(Metadata.java:704)
        at com.travix.supplier.ancillary.service.bundles.interceptors.logging.client.GrpcLoggingClientCallListener.metadataToString(GrpcLoggingClientCallListener.java:81)

What is the reason for this, am I doing something wrong? It seems surprising that the framework would include a key with an illegal value, so I suspect that I implemented it incorrectly.
With some googling I found this piece of code: .java#L66. I tried copying this HTTP_STATUS_LINE_MARSHALLER into my library and create the key with that, but I still got the same exception.
If anyone can tell what I'm doing wrong, and what would be the correct way to implement this, that'd be appreciated!