Minor logging clarification (stream_idle_timeout vs idle_timeout)

I spent some time trying to figure out what was happening with a log entry that featured

```
"response_code_details": "stream_idle_timeout", "duration": 90003,
```

when `stream_idle_timeout` was configured to 15 minutes (90000ms is 1.5 minutes).

Turned out the config field responsible was actually `idle_timeout` as configured per `Route` [here](https://github.com/envoyproxy/envoy/blob/58fce3a7c8b585fd4bfd463497b8dc8a311c46ab/api/envoy/api/v2/route/route_components.proto#L916). (This was non-obvious because the host in question has tens of thousands of routes, so inspecting the config is not trivial.)

Hence feature request: it would be nice if `response_code_details` was more specific about which config limit was actually hit.