Enable instance-level connection

[yu-iskw]

Is your feature request related to a problem? Please describe.
We can't connect an Spanner instance at the time of writing the issue under v1.3.0. We are allowed only to connect a Spanner database in an instance.

Describe the solution you'd like
We would like to connect an instance so that we get a list of databases in a Spanner instance and so on with python-spanner-sqlalchemy.

spanner+spanner:///projects/project-id/instances/instance-id

Describe alternatives you've considered
N/A

Additional context
N/A

[asthamohta]

Hi @yu-iskw, sure we can support this but in this scenario you would need to create a new engine to connect to the database later. Does that work?

[yu-iskw]

That would work for our case!