Provide observation context to exporters

[npepinpe]

Is your feature request related to a problem? Please describe.

I would like to add metrics from my own custom exporter and have them scraped along with the rest of the Zeebe metrics. However, using the default CollectorRegistry.defaultRegistry (or the Micrometer equivalent one) fails, as the exporter is loaded in a different class loader. This also implies that Zeebe will always rely on the default registry, which may not be true forever.

Describe the solution you'd like

Much like the Context provided to the exporter on configuration has a Logger member, I would like the same for metrics. I think using Micrometer (which is to metrics what SLF4J is to logging) would make more sense, but I'm OK with Prometheus :)

Describe alternatives you've considered

You can work around this by loading the CollectorRegistry class directly from the system class loader, e.g.:

final Class<?> registryClass =
    ClassLoader.getSystemClassLoader().loadClass(CollectorRegistry.class.getName());
final Field registryField = registryClass.getDeclaredField("defaultRegistry");
final CollectorRegistry registry = (CollectorRegistry) registryField.get(null);
Counter.build().name("test").namespace("test").register(registry);

As you can see, this is cumbersome, and ignores all the try-catch and possible errors. It also relies on the defaultRegistry global, which makes it harder if we ever more to Micrometer across the board.

[Zelldon]

Triage:

• Moving to backlog, as we don't see an urgency for this
• Please speak up if you think otherwise