Report http.request.bytes and http.response.bytes

**Is your feature request related to a problem? Please describe.**
I'd like to have metrics based on size of requests/responses.

**Describe the solution you'd like**
http.request.bytes and http.response.bytes fields should be set accordingly.

**Describe alternatives you've considered**
Indexing http.headers.content-length; this may not work in all cases.

**Additional context**
https://www.elastic.co/guide/en/ecs/current/ecs-http.html