Disable Startup Time Report for some PR (firebase#4932)

For PR not merge into master branch, Disable Startup Time Report

Author: sunmou99

.github/workflows/health-metrics.yml

@@ -91,7 +91,8 @@ jobs:
  if: |
  (github.event_name == 'push' && github.repository == 'firebase/firebase-android-sdk')
  || (github.event_name == 'pull_request'
- && github.event.pull_request.head.repo.full_name == github.repository)
+ && github.event.pull_request.head.repo.full_name == github.repository
+ && github.event.pull_request.base.ref == 'master')
  runs-on: ubuntu-latest
  steps:
  - uses: actions/checkout@v3