Proposal: add indication in logs whether returned data was in storage or not

**Is your feature request related to a problem? Please describe.**

Most production logs look like this:

```
handler: GET /github.com/arschles/assert/@v/list/ [200]
handler: GET /github.com/arschles/assert/@v/list/ [200]
handler: GET /github.com/arschles/assert/@v/list/ [200]
handler: GET /github.com/arschles/assert/@v/v1.0.0.mod [200]
handler: GET /github.com/arschles/assert/@v/v1.0.0.mod [200]
handler: GET /github.com/arschles/assert/@v/v1.0.0.info [200]
handler: GET /github.com/arschles/assert/@v/v1.0.0.zip [200]
```

It's not really a problem, but read on because we could possibly make these better 😄

**Describe the solution you'd like**

Those logs are nice, but for debugging it can be useful to tell whether the data being returned (for info, mod and zip requests) was from the storage. For `/list` responses, it can also be useful to tell whether the call to the upstream was successful or not (i.e. whether we fell back to storage)

**Describe alternatives you've considered**

I think we could use tracing to get this information instead. Maybe that's just a better choice?

**Additional context**

I'm researching and preparing for #772 and found that some way to tell whether a response was cached or not would be really helpful for us.