Determine default target for broker retry delivery attempts per second

[grantr]

Problem
What is our default target for delivery attempts per second handled by the broker retry service?

This could be defined as a percentage of delivery attempt rate (#1597), for example "10% of delivery attempts will be retried," but since events may be retried multiple times, it's probably clearer to set an explicit rate goal.

Persona:
User

Exit Criteria
We have a goal in delivery attempts per second to tune the default fanout resource requirements.

Additional context (optional)
Part of #1552.

[grantr]

Decided on 10% of total throughput being retried.

[cathyzhyi]

The default target for retry is set to cover 10% failure rate for the fanout delivery. The resulting throughput is sum of a geometric progression with scale factor of 10% and base of 2.5Gb which is fanout target throughput set in #1597 . ( 10%+10%*10% + 10%*10%*10%....)*2.5 Gb/s ~280Mb/s. The target throughput is multiplying payload size * qps * number of triggers so there are scenarios with different combinations from the 3 dimensions when considering the resource request.