beginTransaction (transactionConfig) {
    return this._beginTransaction(this._session._mode, transactionConfig, { api: TELEMETRY_APIS.UNMANAGED_TRANSACTION })
  }
Starts a new explicit transaction with the provided transaction configuration.

@public
@param {TransactionConfig} transactionConfig - Configuration for the new transaction.
@returns {Observable<RxTransaction>} - A reactive stream that will generate at most **one** RxTransaction instance.