feat(GKE): Set the `GITLAB_CONTINUOUS_PROFILING` and `GITLAB_TRACING` environment variables by default.

These environment variables are used by Labkit to configure continuous
profiling and tracing. Setting these variables by default allow these
integrations to "just work", i.e. no additional configuration is required on
the user's end.

The default values can be overwritten by the user, so they can disable
continuous profiling and tracing by adding

```yaml
example-service-k8s:
  spec:
    environment:
      GITLAB_CONTINUOUS_PROFILING: ""
      GITLAB_TRACING: ""
```

to their `gke-service.yaml` file.

Closes: team#662

feat(GKE): Set the `GITLAB_CONTINUOUS_PROFILING` and `GITLAB_TRACING`...

Closes team#662

See merge request !1271