@Nullable
    private CancellationScheduler cancellationScheduler() {
        final ClientRequestContextExtension ctxExt = ctx.as(ClientRequestContextExtension.class);
        if (ctxExt != null) {
            return ctxExt.responseCancellationScheduler();
        }
        return null;
    }
Writes the {@link HttpHeaders trailers} to the {@link Channel}.
Note that the written data is not flushed by this method. The caller should explicitly call
{@link Channel#flush()} when each write unit is done.
core/src/main/java/com/linecorp/armeria/client/AbstractHttpRequestHandler.java