I am trying to understand when it is required to manually configure the session pool.

I read the session docs and it appears to say that only creators of client libraries need to manage sessions manually. However, one way of interpreting that means that I should still set the session pool config appropriately, and the client library will manage the sessions according to what I want to achieve.

In the recent changelog, I see that spanner.NewClient is created with a minimum of 100 opened sessions by default now.

Am I right to say that in many cases the default config will work well, or do I need to use my own config for better performance?

I am also not sure but it seems I used to see Spanner documentation on how to configure the session pool configuration, for example MaxOpened. I can't seem to find that anymore, and the godoc doesn't appear to explain that.