_buildKey(key) {
        if (this.keyPrefix) {
            return `${this.keyPrefix}${key}`;
        }

        return key;
    }
This is a recommended way to build cache key prefixes from
the cache-manager package. Might be a good contribution to make
in the package itself (https://github.com/node-cache-manager/node-cache-manager/issues/158)
@param {string} key
@returns {string}