firebase-ios-sdk

Update merge branch in the Metrics Service (#8146)

GITHUB_REF will get a 'merge' branch due to pull_request , instead of a `master` branch.  
`github.head_ref` could fetch the merged branch, e.g. `master`. 
https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions#github-context