More batch job metadata

The underlying VMs created by GCP Batch have some useful labels like the job ID and WDL task name. These fields are then analyzable in BigQuery, which helps with cost reporting.

We already have `cromwell-sub-workflow-name`, but could we also add a field like `wdl-workflow-name` to represent the top-level workflow? The job submitter's username/email would also be a welcome addition.