Is your feature request related to a problem? Please describe.
The implementation doesn't seem to set a user-agent string to indicate that the requests are coming from SQLAlchemy.

Describe the solution you'd like
Set the user-agent in the dbapi connection.

Describe alternatives you've considered
N/A

Additional context
It seems that dbapi by default sets the DEFAULT_USER_AGENT = "django_spanner/" + VERSION. This may not be ideal.