code: @Nullable private static Attribute<TraceContext> getTraceContextAttributeOrWarnOnce() {
          final RequestContext ctx = getRequestContextOrWarnOnce();
          if (ctx == null) {
              return null;
          }
          return ctx.attr(TRACE_CONTEXT_KEY);
      }
docs: Armeria code should always have a request context available, and this won't work without it.