issue: #37158
Return the GuaranteeTS so that the subsequent requests following the
correct TS.
BeginTS is the current timestamp when the task is created.
The GuaranteeTS is the one parsed based on both consistency level and
beginTS, in PreExecute of the task on Proxy.
The delegator will wait until GuaranteeTS is met.
In PostExecute of the task on Proxy, the TS of the first iterator
request will be returned to the SDK and add it to the subsequent
requests.
Hence, if the default consistency level is Eventually or Bounded, the
order of TS will be
> Guarantee TS < BeginTS
If it returns the BeginTS, the second request will need to catch up and
result in extra 200ms max of latency, which results in something like
| Call | Latency |
| --- | --- |
| first call on `Next()` | 30ms |
| second call on `Next()` | 210ms |
| third call on `Next()` | 10ms |
| fourth call on `Next()` | 11 ms |
| ... | ... |
where we expect
| Call | Latency |
| --- | --- |
| first call on `Next()` | 30ms |
| second call on `Next()` | 10ms |
| third call on `Next()` | 10ms |
| fourth call on `Next()` | 11 ms |
| ... | ... |
Signed-off-by: Patrick Weizhi Xu <weizhi.xu@zilliz.com>
fc69df44a1