[Prometheus] Notify the user when trying to report duplicate metrics

Some libraries happen to report the same metric twice. Although this use case is not clear (to me, see https://github.com/spring-projects/spring-boot/issues/6404) - it happens.

As for spring-boot POV this is OK , it’s clearly not a valid report from Prometheus perspective.

What I suggest is to prevent it while gathering the metrics before the report, notify the user of the error, and prevent the continuation of the report (probably by an exception)

Code reference:
Verify uniqueness of MetricFamilySamples.Sample here: https://github.com/micrometer-metrics/micrometer/blob/5fbaa762afd71b5091e16e165f008ba1e1fe0c18/implementations/micrometer-registry-prometheus/src/main/java/io/micrometer/prometheus/MicrometerCollector.java#L88 and throw an exception if this isn’t a valid report (i.e with duplicate metrics)

[mweirauch]

I can only second Jons statements. How shall we hinder two independant pieces of code from registering the same metric? If these pieces of code do by chance register the same metric, there will never be two metrics registered within the MeterRegistries. (If my naive memory serves me right, that possibility was actually the case in the old Actuator framework, but this isn’t transferable to Micrometer.)

So the proposed sanitization/grouping in the MicrometerCollector has no effect.

The only thing I could think of is hindering registration of an already existing metric, but this will for sure break everything with its current get-or-create-style. (Correct me if I am wrong Jon.)

[jkschneider]

I guess what I was trying to say was that there is no possibility for duplication at this point in the code. Micrometer has already guaranteed name+tag uniqueness at the registry level.

Accidental usage of the same set of name and tags for completely separate purposes in your codebase is not a condition we can detect programmatically. It’s a semantic concern.