async close () {
    this._ssrCallback(this.SSREnabledHint ?? false, 'CLOSE')
    if (this._log.isDebugEnabled()) {
      this._log.debug('closing')
    }

    if (this._protocol && this.isOpen()) {
      // protocol has been initialized and this connection is healthy
      // notify the database about the upcoming close of the connection
      this._protocol.prepareToClose()
    }

    await this._ch.close()

    if (this._log.isDebugEnabled()) {
      this._log.debug('closed')
    }
  }
Call close on the channel.
@returns {Promise<void>} - A promise that will be resolved when the underlying channel is closed.