Support job priority in BigQuery

[sjwhitworth]

Supports the feature discussed in #1456. Haven't managed to test this properly yet, so if you want to merge it, feel free to take it for a spin.

Let me know what you'd like me to do on the documentation side of things as well.

[drewbanin]

This works great @sjwhitworth!

For flags like this, I think it's a good idea to add some quick tests. It's an easy thing to miss in a refactor, and it wouldn't be obvious in manual testing if we introduced a regression here.

Can you add a unit test here? I think you should just be able to set a priority in one of the raw_profile targets, then assert that it comes through in the corresponding Connection object on the other side.

Thanks for making this one!

[drewbanin]

I'd just split this out into a couple of lines. Maybe:

if priority == 'batch':
  job_config.priority = google.cloud.bigquery.QueryPriority.BATCH
else:
  job_config.priority = google.cloud.bigquery.QueryPriority.INTERACTIVE

The else clause probably isn't necessary here, but it's nice to be explicit!

[sjwhitworth]

Sure thing: like I mentioned, I hadn't got round to testing it properly. I'll add this.

Do you want me to add anything on the documentation side?

[sjwhitworth]

Here you go. I think it's not that great a test, given that it doesn't test the actual logic of setting the priority on the job config: just that we pass through the field to the connection obejct. Let me know if you had something else in mind.

I also had to split setting the priority to interactive onto a new line to appease flake8.

[drewbanin]

Great, thanks! I just kicked off the tests here. Don't worry about the docs - I usually touch those up when we cut a release - it's easier to do them all at once, and it's not ideal to document functionality that isn't actually live yet.

I think this test is a-ok. It's unlikely that we'd outright break this code in the connection implementation - instead, these regressions tend to creep in around contract validation. Will merge this one when the tests pass!

Thanks for your contribution!