Getting Values in 15 Min Invervalls with MongoDB Aggregatio

Question

I do have a MongoDB collection that stores values from some Airflow sensor. For putting these values into a line chart, I need these values grouped together into 15 minute intervals. My aggregation query does return the correct values, but I need them to include the timestamp.

The entries in this collection look like this:

{
"_id":{"$oid":"650dd2e15013d08ec65416da"},
"time":{"$numberLong":"1695404767436"},
"tenantId":"someTenantIdString",
"sensorId":"someSensorIdString",
"VolumetricFlowRate":{
    "value":{"$numberDouble":"0.0"},
    "unit":"m3/h",
    "max":{"$numberDouble":"84.21"},
    "mean":{"$numberDouble":"0.025"},
    "min":{"$numberDouble":"0.0"}}

My code for the aggregation function looks like this:

    async getAllValuesFromRange(
        sensorId: string, 
        tenantId: string,
        from: number,
        to?: number, 
    ) {
        const fifteenMinutesInMilliseconds = 1000 * 60 * 15;

        let allValuesFromRange = this.processGasModel
          .aggregate([
              {
                  $match: {
                      time: {
                          $gte: from,
                          ...(to ? { $lte: to } : {})
                      },
                      tenantId,
                      sensorId
                  }
              },
              {
                  $project: {
                      _id: 0,
                      time: -1,
                      lpermin: { $multiply: ["$lpermin", ProcessgasService.DATA_MEASUREMENT_CONVERSION_RATIO] },
                  }
              },
              {
                  $group: {
                      _id: {
                          $toDate: {
                              $subtract: [
                                  "$time",
                                  { $mod: ["$time", fifteenMinutesInMilliseconds] }
                              ]
                          }
                      },
                      totalLiters: { $sum: "$lpermin" },
                      timestamp: { $first: "$time" }
                  }
              },
              {
                  $sort: { _id: 1 }
              },
              {
                $project: {
                    time: "$_id",
                    _id: 0,
                    liters: "$totalLiters"
                }
              },
          ]);
          
          allValuesFromRange.then((result) => {
            console.log("AllValuesFromRange: " + result);
          })


          return allValuesFromRange;
    }

I want the output within 'allValuesFromRange' to look like this: { time: 1718717978632, liters: 483 },{ time: 1718717918630, liters: 275 } where each object represents the grouped values of 15 minutes.