logging/logadmin: ability to add labels

**Is your feature request related to a problem? Please describe.**
I would like to see the ability of adding labels to log-based metrics made possible.

**Describe the solution you'd like**
Would require the addition of metric descriptor and label extractor. two objects that are required to be able to enable labels for log-based metrics.

**Describe alternatives you've considered**
no alternatives exist.

**Additional context**
As per your google documentation - https://cloud.google.com/logging/docs/logs-based-metrics/labels#api