updateTiKVSendReqHistogram is always called with nil resp

[zyguan]

Here we try to pass resp to updateTiKVSendReqHistogram, however we never assign value to the resp since we always return directly in sendRequest (try this). As a result, metrics like TiKVRPCNetLatencyHistogram is actully unavailable.

[sticnarf]

To my understanding, the key is not about whether we assign to resp... Whether we use a closure matters.

Use a closure but no assignment: https://go.dev/play/p/pnM8dhPd4qj

Assign the return variable but don't use a closure: https://go.dev/play/p/YExnvOLOBF2

[zyguan]

Ah, I see, sorry for bothering.