I would like to use Cloud Spanner Emulator to develop this gem and to develop Rails application using gem. Although, there are limitations and differences, the emulator does not cost money 💸 and the separated environments are suited for local development.

https://cloud.google.com/spanner/docs/emulator

Then, I've found out that the current implementation of this library does not support emulator.
This is because Google::Cloud.spanner is used instead of Google::Cloud::Spanner.new. The former method pass the limited portion of parameters to the latter.

If we directly call Google::Cloud::Spanner.new(config), we can pass the emulator_host parameter in config, which enable developers to use Cloud Spanner Emulator.

https://github.com/googleapis/google-cloud-ruby/blob/06c73b3b4b09e5f4dded27b61662fe110fea57bb/google-cloud-spanner/lib/google/cloud/spanner.rb#L91-L101

Although, this breaks config[:project], config[:credentials] compatibility, I think keeping consistency with Google::Cloud::Spanner.new reduces maintenance cost.
Of course, If required, I can write backward-compatible code.

If the above strategy is O.K., I am willing to work on this issue.