private State state(RequestContext ctx) {
        final State state = ctx.ownAttr(stateAttributeKey);
        if (state != null) {
            return state;
        }

        final State newState = new State(numAttrs);
        ctx.setAttr(stateAttributeKey, newState);
        return newState;
    }
Returns a {@link Map} whose key is an export key set through {@code add*()} in
{@link RequestContextExporterBuilder} and value is extracted from the specified {@link RequestContext}.
core/src/main/java/com/linecorp/armeria/common/logging/RequestContextExporter.java