feat(app/metrics): grouped users metrics support

[lowlighter]

Discussed in #1056

Change typing of user/token to allow multiple values:

user: foo, bar
token: |
  {
    "foo": ${{ secrets.FOO_TOKEN }},
    "bar": ${{ secrets.BAR_TOKEN }}
  }

For action:

• Try to test out multiple user data fetching and use adequate merging strategy (probably best to test with a subset of plugins first).
• Instantiate and use octokit with correct token to fetch out private data if asked by user.

For web instances:

• Make a "round-robin" balancing over tokens to support more than 5000 requests