use timestamps, not time since

[xh3b4sd]

I recently read some Prometheus docs. We might do something wrong when emitting the endpoint durations. We emit them as duration. The Prometheus docs recommend emitting timestamps. What do you think @JosephSalisbury

See

• microkit/server/server.go

  Line 220 in dfe4216

   endpointTime.WithLabelValues(endpointMethod, endpointName, endpointCode).Set(float64(time.Since(t) / time.Millisecond)) 

• https://prometheus.io/docs/practices/instrumentation/#timestamps-not-time-since

[Joe Salisbury]

I think there's a small nuance here.

The documentation refers to measuring the time since something took place, and then recommends exporting the unix timestamp - this makes sense.

In our case of endpoint durations, this is the amount of time the endpoint took to complete. I would say exporting the duration makes sense. We can't use time() - my_timestamp_metric to calculate how long it took an endpoint to complete - this would be the time since the endpoint completed.

[xh3b4sd]

Totally makes sense. Vacation brain wasn't certain.