the first requests is very slow in RawKVClient

[zz-jason]

Is your feature request related to a problem? Please describe.

The first request

{
    "start":"17:45:35.667",
    "end":"17:45:35.720",
    "duration":"53ms",
    "func":"get",
    "region":"{Region[8590814505] ConfVer[7164] Version[165] Store[8589948024] KeyRange[xxxxx]:[xxxxx]}",
    "key":"xxxxx",
    "spans":[
        {
            "name":"getRegionByKey",
            "start":"17:45:35.713",
            "end":"17:45:35.715",
            "duration":"2ms"
        },
        {
            "name":"callWithRetry pdpb.PD/GetRegion",
            "start":"17:45:35.713",
            "end":"17:45:35.715",
            "duration":"2ms"
        },
        {
            "name":"gRPC pdpb.PD/GetRegion",
            "start":"17:45:35.713",
            "end":"17:45:35.715",
            "duration":"2ms"
        },
        {
            "name":"callWithRetry tikvpb.Tikv/RawGet",
            "start":"17:45:35.716",
            "end":"17:45:35.720",
            "duration":"4ms"
        },
        {
            "name":"gRPC tikvpb.Tikv/RawGet",
            "start":"17:45:35.716",
            "end":"17:45:35.720",
            "duration":"4ms"
        }
    ]
}

Describe the solution you'd like

make them fast as the client is running for a long time

Describe alternatives you've considered

N/A

Additional context

N/A