@Nullable
    @Override
    public final <V> V setAttr(AttributeKey<V> key, @Nullable V value) {
        requireNonNull(key, "key");
        return attrs.getAndSet(key, value);
    }
Returns the {@link Channel} that is handling this request, or {@code null} if the connection is not
established yet.
core/src/main/java/com/linecorp/armeria/internal/common/NonWrappingRequestContext.java