[discuss] Add number of events per request to metrics data

Knowing the number of events per request would be helpful for troubleshooting, e.g. to give insights into whether one specific app might cause 503 responses and could be tuned to send multiple requests with less events per request instead of one huge request.

Adding that information to the reported metrics, is not straight forward, as it is not just an increasing number to report. For this to be really helpful we'd probably want to store a counter per request and the service name + agent as meta information. I don't believe this is supported in a straight forward way with the currently supported [monitoring metrics types](https://github.com/elastic/beats/blob/master/libbeat/monitoring/metrics.go).

Thanks for starting this conversation @dgieselaar!