info(key) {
        const i = this.#keyMap.get(key);
        if (i === undefined)
            return undefined;
        const v = this.#valList[i];
        const value = this.#isBackgroundFetch(v)
            ? v.__staleWhileFetching
            : v;
        if (value === undefined)
            return undefined;
        const entry = { value };
        if (this.#ttls && this.#starts) {
            const ttl = this.#ttls[i];
            const start = this.#starts[i];
            if (ttl && start) {
                const remain = ttl - (perf.now() - start);
                entry.ttl = remain;
                entry.start = Date.now();
            }
        }
        if (this.#sizes) {
            entry.size = this.#sizes[i];
        }
        return entry;
    }
Get the extended info about a given entry, to get its value, size, and
TTL info simultaneously. Like {@link LRUCache#dump}, but just for a
single key. Always returns stale values, if their info is found in the
cache, so be sure to check for expired TTLs if relevant.