code: static SessionPool createPool(SpannerOptions spannerOptions, DatabaseId db, SpannerImpl spanner) {
      return createPool(
          spannerOptions.getSessionPoolOptions(),
          ((GrpcTransportOptions) spannerOptions.getTransportOptions()).getExecutorFactory(),
          db,
          spanner);
    }
docs: Create a session pool with the given options and for the given database. It will also start
  eagerly creating sessions if {@link SessionPoolOptions#getMinSessions()} is greater than 0.
  Return pool is immediately ready for use, though getting a session might block for sessions to
  be created.