[ENHANCEMENT] Enable Multiplex Spanner Sessions Successfully #1943
Description
We have been excluding the newer version of the spanner SDK because it broke the tests.
It is because of the fact that Multiplex Sessions became enabled by default in the newer versions of the SDK (starting with 1.85.0) and the emulator doesn't seem to handle it well.
For now, we disable it so that it acts like prior to 1.85.0.
We should revisit it and re-enable it by removing the lines in client.go that set GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS and GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW to true and running the tests again.