logging: container_name is not set when using STACKDRIVER appender

We want to use `spring-cloud-gcp-logging` with the `STACKDRIVER` appender from a service that runs in GKE. Our logs go to `projects/my-project-id/logs/spring.log` as expected, including trace and span. However in the `resource.labels` section the `container_name` is not set. All other fields in this section (`cluster_name`, `location`, `namespace_name`, `pod_name`, `project_id`) are all set to the expected values.

Is this potentially a bug in the logging library or simply a quirk of how Stackdriver works when not logging to STDOUT? If not a bug, is it possible to manually set this field via some logger configuration? Our intention was to use the `container_name` in our log search filters.

- Spring Boot: 2.6.6
- Spring Cloud 2021.0.1
- Spring Cloud GCP: 3.2.1

I'm aware of the `CONSOLE_JSON` appender, which works great, but we have an unusual business case where we need unfiltered 'standard' logs as well as structured logs.

Start by reproducing the STACKDRIVER appender behavior in a service running in GKE and inspect the resulting resource.labels, especially container_name. Compare it with the CONSOLE_JSON appender and determine whether the missing label comes from the logging library or Stackdriver; done means documenting or enabling a supported way to populate the field, if one exists.

Written by the indexing model from the issue text.