Resource control: The RU consumption by TiFlash is not show in grafana but it impacts the available ru

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

One of our customer saw the phenomenon that will the RU consumption by 1 resource group is very low but when the use change the resource group setting from burstable to not burstable, the latency has increased a lot.

After some investigation, we found that the low RU usage is due to most of ru is consumed by tiflash which is not presented on that page.

Two potential approaches to handle this issue:
1) optimize tiflash's resource control behavior to let it similar to tikv.
2) Make tiflash's behavior like background ru management, that is only count its ru consumption but does not affect the ru control at tidb side.

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

### 3. What did you see instead (Required)

### 4. What is your TiDB version? (Required)