Set<String> getLabels() {
            return topics.stream()
                    .flatMap(topic -> topic.getLabels().stream().map(Topic.Level::getContent))
                    .collect(Collectors.toSet());
        }
Validates that labels are correct for MQTT topics on
{@code subscribe} and {@code publish} operations.

<ul>
    <li>Operation input is required when a topic has labels.</li>
    <li>Topic labels must be found in the input.</li>
    <li>The input must not contain extraneous labels.</li>
</ul>
smithy-mqtt-traits/src/main/java/software/amazon/smithy/mqtt/traits/validators/MqttTopicLabelValidator.java