RateLimiterMongo: add option not to create indexes (a must for for serverless environments)

[tomyam1]

We use RateLimiterMongo on a server running on AWS lambda.

Since RateLimiterMongo calls createIndex on creation, it results in calling createIndex for every lambda invocation that is a cold start.

This has two issues:

1. Causes lambda to crash with UnhandledPromiseRejection, for example

Unhandled Promise Rejection 	{"errorType":"Runtime.UnhandledPromiseRejection","errorMessage":"MongoError: Pool was force destroyed","reason":{"errorType":"MongoError","errorMessage":"Pool was force destroyed","name":"MongoError","stack":["MongoError: Pool was force destroyed","    at Pool.destroy (/var/task/node_modules/mongodb/lib/core/connection/pool.js:679:21)","    at Server.destroy (/var/task/node_modules/mongodb/lib/core/topologies/server.js:902:15)","    at Timeout._onTimeout (/var/task/node_modules/mongodb/lib/core/topologies/replset.js:357:26)","    at listOnTimeout (internal/timers.js:554:17)","    at processTimers (internal/timers.js:497:7)"]},"promise":{},"stack":["Runtime.UnhandledPromiseRejection: MongoError: Pool was force destroyed","    at process.<anonymous> (/var/runtime/index.js:35:15)","    at process.emit (events.js:326:22)","    at process.EventEmitter.emit (domain.js:483:12)","    at processPromiseRejections (internal/process/promises.js:209:33)","    at processTicksAndRejections (internal/process/task_queues.js:98:32)"]}

This is because createIndex is called without waiting for the result. If the its called in one lambda call and it finished before its done it will throw in the following lambda invocation.

2. Performance

I would recommend:

1. Adding option to choose if createIndex should be called at all
2. Add a method public method to call createIndex with async
3. Catch errors from the createIndex calls in _initCollection with rethrow. It's very difficult to find the reason for the UnhandledPromiseRejection otherwise.

This is what we currently use to alleviate the issue:

/**
 * Rate limiter that doesn't create indexes on initialize
 * Instead we create the indexes once on init
 */
export class RateLimiterMongoose extends RateLimiterMongo {
    public readonly tableName: string;
    protected client: Connection;
    protected _collection: Collection

    /**
     * Override _initCollection and don't call createIndex
     *
     * Source: https://github.com/animir/node-rate-limiter-flexible/blob/v2.2.1/lib/RateLimiterMongo.js#L78
     */
    protected _initCollection(): void {
        this._collection = this.client.collection(this.tableName);
    }

    /**
     * Create the indexes required, but unlike the original _initCollection, use await to catch for errors
     */
    public async initIndexes(): Promise<void> {
        const collection = this.client.collection(this.tableName);
        await collection.createIndex({ expire: -1 }, { expireAfterSeconds: 0 });
        await collection.createIndex(Object.assign({}, this.indexKeyPrefix, { key: 1 }), { unique: true });
    }
}

[Systerr]

This will be useful for testing too.
In our case we are using mongodb-memory server for testing and this affect us too