Missing class SpelValueExpressionResolver from documentation

[a-simeshin]

When configuring Micrometer to use @Metertag annotation according to the documentation micrometer dependencies missing class SpelValueExpressionResolver even in spring-boot application.

1. I propose to add some information into documentation that will describe that SpelValueExpressionResolver should be implemented independently atm or should be requested in spring-boot project for spring-boot applications.
2. In addition, I propose to create example of simple implementation in documentation that does not use SPEL for cases when the application does not need to calculate the values ​​of tags through SPEL at all.

Rationale
According to the documentation code snippet does not work cause class SpelValueExpressionResolver is missing.

ValueResolver valueResolver = parameter -> "Value from myCustomTagValueResolver [" + parameter + "]";
// Example of a ValueExpressionResolver that uses Spring Expression Language
ValueExpressionResolver valueExpressionResolver = new SpelValueExpressionResolver();
// Setting the handler on the aspect
timedAspect.setMeterTagAnnotationHandler(
        new MeterTagAnnotationHandler(aClass -> valueResolver, aClass -> valueExpressionResolver));

Additional context
Im facing this issue with:

• spring-boot 3.5.0
• micrometer 1.15.0

After some research I found that private class exists SpelTagValueExpressionResolver for the spring-boot applications with a bit misspelling name:

• SpelTagValueExpressionResolver in the spring-boot
• SpelValueExpressionResolver in the micrometer documentation

Unfortunately, there is no bean for this class and single usage is here as part of SpanTagAnnotationHandler. May be this class can be created as bean for Timed and Counted aspects as additional changes in spring-boot.

[jonatan-ivanov]

I think we should add a not/comment to the docs that says that SpelValueExpressionResolver is a non-existing example class that you need to implement for your framework (if it does not already provide one). Maybe we should also drop the "spel" part and just call it CustomValueExpressionResolver? Would that work in your case? If so, would you be interested fining a PR (against 1.13.x)?

[a-simeshin]

@jonatan-ivanov, thanks for the feedback! I will be glad to create a PR, because MeterTag is incredibly useful and reducing meter code by a lot.

Let's clarify what is the correct thing to do:

1. Rename SpelValueExpressionResolver to CustomValueExpressionResolver.
2. Add note that CustomValueExpressionResolver actually does not exist and should be implemented if it does not already provided by the application's frameworks.
3. Should I add example of implementations SpelValueExpressionResolver as additional example for spring-boot applications?

Also I'd like to ask your opinion on whether should I start issue in spring-boot project to make SpelTagValueExpressionResolver public and provide this one as bean for current TimedAspect and in the future for CountedAspect to enable MeterTag by default?

[jonatan-ivanov]

1. Yes.
2. Yes.
3. I'm not sure, since Spring Boot has one. Maybe we can link to the Boot docs?

Also I'd like to ask your opinion on whether should I start issue in spring-boot project [...]

Please check this first spring-projects/spring-boot#46007 and feel free to comment it/leave your feedback.