Support configuring additional properties on SQLAlchemy engine

[rolyv]

CloudSQL documentation on managing connections (https://cloud.google.com/sql/docs/postgres/manage-connections) gives guidance on managing connection pool size and connection timeout/duration. Code sample here: https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/cloud-sql/postgres/sqlalchemy/main.py

Pasting important part here:

# [START cloud_sql_postgres_sqlalchemy_create]
# The SQLAlchemy engine will help manage interactions, including automatically
# managing a pool of connections to your database
db = sqlalchemy.create_engine(
    # Equivalent URL:
    # postgres+pg8000:https://<db_user>:<db_pass>@/<db_name>?unix_sock=/cloudsql/<cloud_sql_instance_name>/.s.PGSQL.5432
    sqlalchemy.engine.url.URL(
        drivername='postgres+pg8000',
        username=db_user,
        password=db_pass,
        database=db_name,
        query={
            'unix_sock': '/cloudsql/{}/.s.PGSQL.5432'.format(
                cloud_sql_connection_name)
        }
    ),
    # ... Specify additional properties here.
    # [START_EXCLUDE]

    # [START cloud_sql_postgres_sqlalchemy_limit]
    # Pool size is the maximum number of permanent connections to keep.
    pool_size=5,
    # Temporarily exceeds the set pool_size if no connections are available.
    max_overflow=2,
    # The total number of concurrent connections for your application will be
    # a total of pool_size and max_overflow.
    # [END cloud_sql_postgres_sqlalchemy_limit]

    # [START cloud_sql_postgres_sqlalchemy_backoff]
    # SQLAlchemy automatically uses delays between failed connection attempts,
    # but provides no arguments for configuration.
    # [END cloud_sql_postgres_sqlalchemy_backoff]

    # [START cloud_sql_postgres_sqlalchemy_timeout]
    # 'pool_timeout' is the maximum number of seconds to wait when retrieving a
    # new connection from the pool. After the specified amount of time, an
    # exception will be thrown.
    pool_timeout=30,  # 30 seconds
    # [END cloud_sql_postgres_sqlalchemy_timeout]

    # [START cloud_sql_postgres_sqlalchemy_lifetime]
    # 'pool_recycle' is the maximum number of seconds a connection can persist.
    # Connections that live longer than the specified amount of time will be
    # reestablished
    pool_recycle=1800,  # 30 minutes
    # [END cloud_sql_postgres_sqlalchemy_lifetime]

    # [END_EXCLUDE]
)
# [END cloud_sql_postgres_sqlalchemy_create]