How to use the apollo-server-caching.PrefixingKeyValueCache function in apollo-server-caching

} else if (extensions.persistedQuery.version !== 1) {
      throw new InvalidGraphQLRequestError(
        'Unsupported persisted query version',
      );
    }

    // We'll store a reference to the persisted query cache so we can actually
    // do the write at a later point in the request pipeline processing.
    persistedQueryCache = config.persistedQueries.cache;

    // This is a bit hacky, but if `config` came from direct use of the old
    // apollo-server 1.0-style middleware (graphqlExpress etc, not via the
    // ApolloServer class), it won't have been converted to
    // PrefixingKeyValueCache yet.
    if (!(persistedQueryCache instanceof PrefixingKeyValueCache)) {
      persistedQueryCache = new PrefixingKeyValueCache(
        persistedQueryCache,
        APQ_CACHE_PREFIX,
      );
    }

    queryHash = extensions.persistedQuery.sha256Hash;

    if (query === undefined) {
      query = await persistedQueryCache.get(queryHash);
      if (query) {
        metrics.persistedQueryHit = true;
      } else {
        throw new PersistedQueryNotFoundError();
      }
    } else {
      const computedQueryHash = computeQueryHash(query);

};
      }
    }

    if (!requestOptions.cache) {
      requestOptions.cache = new InMemoryLRUCache();
    }

    if (requestOptions.persistedQueries !== false) {
      const {
        cache: apqCache = requestOptions.cache!,
        ...apqOtherOptions
      } = requestOptions.persistedQueries || Object.create(null);

      requestOptions.persistedQueries = {
        cache: new PrefixingKeyValueCache(apqCache, APQ_CACHE_PREFIX),
        ...apqOtherOptions,
      };
    } else {
      // the user does not want to use persisted queries, so we remove the field
      delete requestOptions.persistedQueries;
    }

    this.requestOptions = requestOptions as GraphQLServerOptions;

    if (uploads !== false && !forbidUploadsForTesting) {
      if (this.supportsUploads()) {
        if (!runtimeSupportsUploads) {
          printNodeFileUploadsMessage();
          throw new Error(
            '`graphql-upload` is no longer supported on Node.js < v8.5.0.  ' +
              'See https://bit.ly/gql-upload-node-6.',

requestDidStart(
      outerRequestContext: GraphQLRequestContext,
    ): GraphQLRequestListener {
      const cache = new PrefixingKeyValueCache(
        options.cache || outerRequestContext.cache!,
        'fqc:',
      );

      let sessionId: string | null = null;
      let baseCacheKey: BaseCacheKey | null = null;
      let age: number | null = null;

      return {
        async responseForOperation(
          requestContext,
        ): Promise {
          requestContext.metrics.responseCacheHit = false;

          if (!isGraphQLQuery(requestContext)) {
            return null;

constructor(
    keyValueCache: KeyValueCache = new InMemoryLRUCache(),
    httpFetch: typeof fetch = fetch,
  ) {
    this.keyValueCache = new PrefixingKeyValueCache(
      keyValueCache,
      'httpcache:',
    );
    this.httpFetch = httpFetch;
  }