Event.duration is often inappropriate, missing http request+response+ ... durations

We have several use cases with http response time etc. We use event.duration here. But sometimes we need to store:
- time taken to send the http request
- time taken to receive the http response
- time taken by client to process the received html page
- total time

The total time here is mapped to event.duration, but there are no other fields. So I propose to add at least:
- http.request.duration (is usually event.duration now, but should be sum of all times in the event)
- http.response.duration