Resource control quota maybe exceed in some scenarios

[lilinghai]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

some workloads run together and set ru limit
tpcc 80k limit and the workload fluctuates around 80k, with a maximum reaching 88k.
It is somewhat related to the current implementation. When a request comes in, the RU consumption is added first, rather than after wait. In the case of tpcc, the write RU consumption is relatively high with request. It indicates a small peak, exceeding 10%.

2. What did you expect to see? (Required)

resource usage is under ru limit

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master

[nolouch]

it can't go too far beyond the specified RU rate, so I think that's reasonable. Let me lower the bug level first.

[nolouch]

It meets the expectations in the current implementation.