Add SlotMillis to QueryStatistics.
Add SlotMillis to QueryStatistics.

Now, total slot millisecond usage is reported for job, in addition
to the cumulative updates as part of the query timeline.

Change-Id: I8bcd1f32dd25e752bf0194097fff304d3d764f67
Reviewed-on: https://code-review.googlesource.com/27430
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go