Fix duplicated size metrics in Datadog

[lucia-sb]

What does this PR do?

Updates the measure_disk_size workflow to determine the branch from github.event.workflow_run.head_branch instead of github.ref.

Motivation

Metrics were being duplicated in Datadog because all workflow runs were reported under master.
This happened because github.ref refers to the branch where the workflow itself runs — and for workflow_run triggers, that’s always master. By switching to github.event.workflow_run.head_branch, the workflow now correctly compares the branch that actually triggered the run, avoiding duplicated metrics.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[AAraKKe]

Nice catch!! Thanks a lot. Lets get it merged and see if that was the issue.