public static void copy(RequestContext ctx, String key) {
        requireNonNull(ctx, "ctx");
        requireNonNull(key, "key");
        checkState(delegate != null, ERROR_MESSAGE);
        put(ctx, key, delegate.get(key));
    }
Copies the specified thread-local {@link MDC} property to the specified {@link RequestContext}.

@param ctx the {@link RequestContext}
@param key the key of the thread-local {@link MDC} property to copy
core/src/main/java/com/linecorp/armeria/common/logging/RequestScopedMdc.java