Explicitly set counter values

[rauhul]

swift-metrics doesn't currently provide a Gauge.record api for Counters and this probably makes as Counters are supposed to be strictly incrementing. However this behavior doesn't compose if the metric source already accumulates the value.

let total = GetTotalNetworkPackets()
Counter(label: "network_packets_total")
  .increment(total) // <-- increment is wrong here (double counting)

Instead you have to use a Gauge to directly set this value. However consumers now see the value as a gauge and not a counter which is the real behavior.

I'm not sure if this is actually something we want to support, but it probably warrants some thought.

---

Prometheus in go provides an escape hatch for this using NewConstMetric

[ktoso]

Thanks for reporting, I have somewhat mixed feelings about it.

On one hand, a Counter is really defined as being "grow only", so allowing a record API means we would probably need to enforce that grown-only nature...

• Should we crash if someone reports 10 and then 1 though? Seems a bit too "intense", for a metrics library.
• Should we just pass through to the underlying library but warn implementations about this?

The prom API you mention also does not allow for directly recording on a Counter but offers this escape hatch... Unless we really have to, I think we should avoid doing this in this API package; Actual metrics libs can offer those hatches if necessary.

Let me know though if this would be a huge blocker for you -- so far I understand you can get away with the Gauge approach.