metrics: use Application Default Credentials (ADC) with attached service account for Cloud Run

[trivikr]

Is your feature request related to a problem? Please describe.

I couldn't come across any documentation, when diving deep on #3697
While doing peer programming with @targos, I ended up copying metrics-processor-service-key.json from previous Dockerfile mentioned in #3767 (comment)

However, there's a better way of providing Application Default Credentials (ADC) using already available service account [email protected]

Describe the solution you'd like

Attached service account when deploying Cloud Run

gcloud run deploy processlogs --image gcr.io/nodejs-org/processcloudflare:latest --service-account [email protected] --region us-central1 --no-allow-unauthenticated

Remove steps for copying metrics-processor-service-key.json, and create Client without any params.