@Nullable
    public static String get(RequestContext ctx, String key) {
        requireNonNull(ctx, "ctx");
        requireNonNull(key, "key");

        final String value = getMap(ctx).get(key);
        if (value != null) {
            return value;
        }

        final RequestContext rootCtx = ctx.root();
        if (rootCtx != null && rootCtx != ctx) {
            return getMap(rootCtx).get(key);
        }

        return null;
    }
Returns the value of the specified request-scoped {@link MDC} property bound to the specified
{@link RequestContext}.

@param ctx the {@link RequestContext}
@param key the key of the request-scoped {@link MDC} property

@return the request-scoped {@link MDC} property. {@code null} if not found.
core/src/main/java/com/linecorp/armeria/common/logging/RequestScopedMdc.java