Provide a way to set a `Counter` to an absolute value

[K900]

I'm writing a reexporter for another service that already outputs aggregated metrics (but not in a Prometheus compatible format). It would be very convenient to be able to directly call .set(existing_value) on a Prometheus counter, instead of having to track the previous value manually and inc_by a computed amount.

[mxinden]

Would a const counter solve your use-case?

https://docs.rs/prometheus-client/latest/prometheus_client/metrics/counter/struct.ConstCounter.html

[K900]

AFAIUI const counters just can't be changed at all? I want to be able to change the value, I just already have it pre-aggregated.

[mxinden]

You would use the Collector trait to implement your own metrics collector. On a scrape of your exporter by Prometheus, prometheus_client would call Collector::collect. Your collector implementation would retrieve the metric values from your actual monitoring target, and expose the values via e.g. ConstCounter. See Collector docs for details.

Hope this is helpful.

[K900]

Hmm, this would require a lot of refactoring, but I think it could work.

[K900]

Actually, maybe not... Getting values from the target is async right now, and Collector's interface is sync, so I'd have to block...

[K900]

OK so some refactoring later, it looks like I can't really do this without Collector being async, or using my own atomics or whatever to store values...

[mxinden]

Getting values from the target is async right now, and Collector's interface is sync, so I'd have to block...

@K900 is that a problem? I assume you are running your /metrics http endpoint on a separate OS thread. Prometheus scrapes an enpoint on the order of seconds. I assume the latency to your target is low. Why is blocking on a future an issue here?

[K900]

Mostly just because I'm calling another API client that's already async. I can probably run it on another thread, but so far everything has been part of one big event loop.

[sialais]

I've encounterd with the same problem. and i found it could use the inner() method to get the reference of value, and set it.
for example

let counter = Counter::<u64, AtomicU64>::default();
counter.inner().set(1234);

[K900]

Oh, that's kinda cursed but I guess it should work?

[koushiro]

@sialais @K900 if you want to set the counter metric, why not using gauge instead of counter?
Semantically, counter cannot be set, only incremented.

[K900]

This is mostly convenient for writing reexporters, where you're just converting data from some external thing to Prometheus format.