Warn when a MeterFilter is configured after a meter is registered

[shakuzen]

In normal usage we would expect this should not happen, but there is nothing enforcing or encouraging it to not happen. This warns users via logging that it is happening and advises to correct it, if possible. Otherwise, we want to learn about why it is not fixable. The problem with configuring MeterFilters after a Meter has been registered is that such filters will not be applied to previously registered meters. This can result in a mix of metrics with all filters applied and only some filters applied, which is potentially hard to notice.

[lenin-jaganathan]

FFT:

1. Should we deal with the composite meter registries differently?
2. How would this likely affect the parent and child registries and the addition of meter filters in composite registries.

[shakuzen]

Thanks for the review. I didn't particularly think about composite registries. What kind of scenarios are you thinking might require special attention?

[lenin-jaganathan]

Thanks for the review. I didn't particularly think about composite registries. What kind of scenarios are you thinking might require special attention?

One particular case I am aware of is with Metrics.globalRegistry. We use spring-boot actuator for wiring up the registries and then add them to the global one. There are third-party libraries that add metrics directly to the global registries and sometimes this happens before we add any meter filters.

Why we add filters to global registry?
This is to avoid overusing memory when third party libraries / users add meters directly to the global registry.

[shakuzen]

Thanks for the feedback. That's good to be aware of.

There are third-party libraries that add metrics directly to the global registries and sometimes this happens before we add any meter filters.

If whatever is configuring everything (e.g. Spring Boot auto-config) can be made to not do what will result in registering meters until after filters have been configured, this should be solvable. It's just made harder by there being a "hidden" dependency on the global MeterRegistry that e.g. Spring Boot doesn't and can't automatically know about.

It's not clear what we can/should change about this pull request for this yet, so let's get this into RC1 and collect feedback. Let's try to figure out as many of these scenarios as we can in the upcoming month before GA based on testing with the RC1 version. Hopefully we can find a good solution for these.