private Optional<FailoverContext[]> getContexts(MetadataDelta delta) {
        return Optional.ofNullable(delta.kvDelta())
            .map(KVDelta::changedKV)
            .map(kv -> kv.get(FailoverConstants.FAILOVER_KEY))
            .map(this::decodeContexts);
    }
Get the failover contexts from the given metadata delta.
It returns empty if there is no update for the failover key.
core/src/main/java/kafka/automq/failover/FailoverListener.java