Add debug benchmarking time spent in fetch and fetch_multi

Add an extra log around cache `fetch` and `fetch_multi` in order to better diagnose time spent performing the actions.

This is done via the help of a module helper
```
add_benchmark_to_method :fetch
```

Here is the log sample:
```
[IdentityCache] call_time=0.24 ms
```

This is performed in the log level `DEBUG`

@airhorns @camilo @fbogsany thoughts?