Good question - the question of whether to log the millisecond when the request starts or ends is a great example of how complex these things are to think about accurately, let alone capture.

I'd want to log when the requests start, as I'm mostly concerned with how well-distributed request arrival was at that level of granularity.

I wondered if the network layers in between my client and server were effectively "smoothing" request arrival across each second, or if instead requests were very bursty so that a per-minute spike in a typical graph was dominated by a few seconds or milliseconds within that minute.