clear(callback) {
    Promise.all(
      this.aggregators.map((aggregator) => {
        return new Promise((resolve) => {
          if (aggregator.enabled) {
            aggregator.once(`finished_data_send-${aggregator.method}`, function finish() {
              resolve()
            })

            aggregator.send()
          } else {
            // No data to flush because aggregator is not enabled
            resolve()
          }
        })
      })
    ).then(() => {
      // Get out of the promise so callback errors aren't treated as
      // promise rejections.
      setImmediate(callback)
    })
  }
Calls send on every registered aggregator that is enabled.
It then will wait for the collector to acknowledge that the data has been
sent and resolve a promise.

@param {Function} callback function to call once all aggregators have sent data