Add tests and docs for every exposed metric

[lidel]

To avoid bugs like ipfs/boxo#162 we need to have regression test for every gateway metric fro go-libipfs.

We should also have ./docs/METRICS.md which explains what is exposed and why (reuse notes from https://www.notion.so/pl-strflt/Rhea-Project-Metrics-55f08f7b33564dc59926d63c282ab564 for now, we can refine it later)

[BigLep]

Yes, definitely agreed on needing to have our metrics documented.

One area I often see get missed is around latency metrics. Please be clear on "when is the stopwatch started" and "when is the stopwatch ended".

[hacdias]

@lidel perhaps this tests (and docs) would be better to live in go-libipfs instead, with the gateway code itself? I will investigate how to add tests for the metrics.