franz-go: kotel: error while marshaling: string field contains invalid UTF-8

With the kotel tracing plugin enabled, try producing a record where the record contains an invalid utf-8 sequence. It ends up dropping the span.

client.ProduceSync(ctx, &kgo.Record{
	Key:   []byte{0xe2, 0x28, 0xa1},
	Value: []byte{},
	Topic: topic,
})

traces export: rpc error: code = Internal desc = grpc: error while marshaling: string field contains invalid UTF-8

I’m happy to submit patches, just making the issue known.