getDataChannelCount () {
    return Object
      .values(this._channelMap)
      .filter(c => _includes(DATA_CHANNEL_TYPES, c.channel))
      .length
  }
Get the total number of data channels this instance is currently
subscribed too.

@returns {number} count
@see WSv2#subscribeTrades
@see WSv2#subscribeTicker
@see WSv2#subscribeCandles
@see WSv2#subscribeOrderBook