connect = async () => {
  if (!credConfig) credConfig = getCredConfig();
  if (process.env.DB_HOST) {
    return connectWithTcp(credConfig);
  } else {
    return connectWithUnixSockets(credConfig);
  }
}
Connect to the Cloud SQL instance through TCP or UNIX Sockets
dependent on DB_HOST env var

@returns {object} Knex's PostgreSQL client