Add default factoryType tag in CommonsObjectPool2Metrics

[HYEONSEOK1]

Overview

This PR addresses the issue of Prometheus tag collision by introducing distinct metric name prefixes for different pool types in the CommonsObjectPool2Metrics class. This change ensures that metrics from GenericObjectPool and GenericKeyedObjectPool have unique names, preventing conflicts caused by differing tag sets.

This PR addresses the issue of Prometheus tag collision by ensuring that all metrics have a consistent set of tags. For GenericObjectPool, a factoryType tag is added, and for other pool types, a default value of factoryType=none is used. This approach resolves the conflict caused by differing tag sets between GenericObjectPool and other pool types.

Changes

- Introduced distinct metric name prefixes: objectpool. for GenericObjectPool and keyedobjectpool. for GenericKeyedObjectPool.
- Updated the registerMetricsEventually method to apply the appropriate prefix based on the pool type.
- Adjusted metric registration methods (registerGaugeForObject, registerFunctionCounterForObject, registerTimeGaugeForObject) to use the new prefixes.

• Added a default factoryType=none tag for all pool types except GenericObjectPool.

Reasoning

Prometheus requires that all meters with the same name have the same set of tag keys. The previous implementation violated this rule by adding a factoryType tag for GenericObjectPool, which was not present in other pool types. This caused registration failures in PrometheusMeterRegistry at this line. By ensuring that all metrics include a consistent set of tags, we eliminate tag collisions and comply with Prometheus' requirements.

[jonatan-ivanov]

Thank you for the PR!
Since in Micrometer we consider changing existing meter names and tags a breaking change, I don't think we should do this outside of a new major version.
What we can do though is assigning a value to factoryType if it is not available, e.g.: none. What do you think about doing that instead?

[jonatan-ivanov]

Also, for everyone who might bump into this issue: you can work this around by adding a MeterFilter that adds a factoryType=none tag if it does not exists for these metrics.

[HYEONSEOK1]

Thank you for your feedback and suggestions!

I’ve reverted the previous changes and updated the code as suggested. The nameTag method now defaults to factoryType=none for all pool types except GenericObjectPool, which gets its specific factoryType tag.

[jonatan-ivanov]

I polished your changes a bit and added tests so that we verify that all the tags are there all the time and rebased it on 1.9.x so that older versions of Micrometer will also get the fix.