Make io.prometheus.client.CollectorRegistry.defaultRegistry usable together with iapetos

[delitescere]

G'day! This is an awesome library. Thank you so much for making it.

I'm enjoying the ring and jvm wrappers very much. However, I've recently added to my project the InstrumentedAppender but it only has a mechanism for registering its collector with the Prometheus Java client's default registry.

This means that the ring wrapper exposing metrics at /metrics doesn't pick up any non-iapetos collectors because a different default registry is created by iapetos.

I'm wondering if https://github.com/xsc/iapetos/blob/master/src/iapetos/registry.clj#L79 should be more like:

(defn create
  ([]
   (IapetosRegistry. "iapetos_registry" (CollectorRegistry/defaultRegistry) {} {})))
  ([registry-name]
   (IapetosRegistry. registry-name (CollectorRegistry.) {} {})))

This keeps the "iapetos_registry" name for the default registry for any existing code that might use it, but uses the underlying Prometheus Java client's default registry.

[xsc]

@delitescere Thanks for reaching out! I do have a problem, though, with implicitly exposing the default registry, since create kinda indicates that a fresh registry is produced. What do you think about just adding iapetos.core/default-registry? Then anyone could decide to use it directly and explicitly.

Note to self: pushable-collector-registry needs an adjustment for this, allowing a registry to be injected.

[delitescere]

@xsc excellent idea.

[xsc]

Release Notes

[delitescere]

Yes, the problem is now there's no surface idempotency. I'm using this in a stuartsierra system, so components can be started, stopped, and restarted. As the same registry is being used, the collectors have already been registered. There's no iapetos mechanism for unregistering at the moment.

Given I'm using the Prometheus default registry, I can add the following to the "stop" lifecycle task of the component:

(.clear CollectorRegistry/defaultRegistry)

This is a little heavy-handed as any collectors my Clojure app doesn't register as part of the "start" lifecycle will be unregistered and not return (although this is not likely in my case). Perhaps the IapetosRegistry protocol could forward the clear and unregister methods to the underlying Prometheus registry?

On sum, though, I'm perfectly happy with things as they are. Many thanks!