feat: support request and transaction tags

[olavloite]

Do not merge: Depends on googleapis/python-spanner#1262

Fixes #525

[ahr9n]

Thanks for your support!

BTW, this request_tag = self.execution_options.get("request_tag") means the request tag will be on connecting to spanner engines [so, it will be the same for all the transaction queries -- on transaction level], but it should be on executing queries [to be on query level], right?

transaction_tag = self.execution_options.get("transaction_tag") is making sense, since the whole transaction will belong to one connection.

[olavloite]

@ahr9n

BTW, this request_tag = self.execution_options.get("request_tag") means the request tag will be on connecting to spanner engines [so, it will be the same for all the transaction queries -- on transaction level], but it should be on executing queries [to be on query level], right?

You can set execution_options on a lower level than the engine. E.g. it can also be set for specific queries like this:

with Session(engine.execution_options(read_only=True)) as session:
    # Execute two queries in a read-only transaction.
    session.scalars(
        select(Singer).execution_options(request_tag="my-tag-1")
    ).all()
    session.scalars(
        select(Singer).execution_options(request_tag="my-tag-2")
    ).all()

The request tag is then set on the cursor that is being used for the current query:

request_tag = self.execution_options.get("request_tag")
if request_tag:
    self.cursor.request_tag = request_tag