Content-Length omitted by OkHttp so payload size is not reported in Firebase

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### [REQUIRED] Step 2: Describe your environment

* Android Studio version: 3.5.1
* Firebase Component: Performance
* Component version: 19.1.0

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

When using Firebase Performance Monitoring (automatic http metrics) with OkHttp, the response payload size is not reported to Firebase.

The reason is if OkHttp does transparent gzip decompression, the library omits the `Content-Length` header, and hence Firebase is unable to report it.

I do not know what would be better -

- Intercept requests at network level rather than application level
- Check body.length instead of `Content-Length`

Here is more details about the issue on OkHttp
https://github.com/square/okhttp/issues/259#issuecomment-547986658

#### Relevant Code: