runner example for `--cloud-metadata`

[dacbd]

Thoughts on some help tags/examples for users.

for example; in with a GitHub actions example

cml runner ... \
  --cloud-metadata="repo=${{ github.repository }}"
  --cloud-metadata="ActionsRun=https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}"
  ...

[casperdcl]

From this, we should also document --cloud-metadata="Key with space=value with space" in https://cml.dev/doc/ref/runner