feat: add pod startup metrics

[w21froster]

Description

Type of change

• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] New feature / enhancement (non-breaking change which adds functionality)
• [ ] Security fix
• [ ] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [X] Add changelog entry following the contributing guide
• [ ] Documentation has been updated
• [ ] This change requires changes in testing:
  • [x] unit tests
  • [ ] E2E tests

[CLAassistant]

All committers have signed the CLA.

[csongnr]

ran e2e tests here : https://github.com/newrelic/nri-kubernetes/pull/969 because e2e testing doesn't work w/ prs from forked repos. all tests passed, so will be merging