Github user m-hogue commented on the issue:
https://github.com/apache/nifi/pull/2150
Since there's not a clean way to unit test this, the way i tested was to
change unit tests and observe that there were etag cache hits until `max-age=1`
seconds had passed, when a network request would be issued again. For example:
```
2751 [pool-2-thread-1] DEBUG org.apache.nifi.processors.standard.InvokeHTTP
- InvokeHTTP[id=6a72be7c-560d-4bdf-b58e-a59aaaec7478] OkHttp ETag cache metrics
:: Request Count: 99 | Network Count: 2 | Hit Count: 97
```
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