prometheus
Provide a way to set a `Counter` to an absolute value
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.
Would a const counter solve your use-case?
https://docs.rs/prometheus-client/latest/prometheus_client/metrics/counter/struct.ConstCounter.html
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.
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.
Actually, maybe not... Getting values from the target is async right now, and Collector's interface is sync, so I'd have to block...
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...
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?
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.
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);
@sialais @K900 if you want to set the counter metric, why not using gauge instead of counter? Semantically, counter cannot be set, only incremented.
This is mostly convenient for writing reexporters, where you're just converting data from some external thing to Prometheus format.