Updates list of exposed metrics

[Ashiroq]

Closes #1458
Not entirely sure if this is correct.

[hiddeco]

Thanks a lot for your contribution! 🏅

It is a great start but I think it would be even better if we would make it a table in the following format:

| metric                                | description                             |
|---------------------------------------|-----------------------------------------|
| `flux_cache_request_duration_seconds` | Duration of cache requests, in seconds. |

This way a user not only has a description of what is exposed but also knows how it is exposed.

[dholbach]

Excellent work @Ashiroq and thanks for the review @hiddeco!

[squaremo]

Yay! Thanks @Ashiroq