Create baggage scope for method using annotations

[stuartkemp-e]

Please describe the feature request.
I would like to be able to put values into baggage scope for the duration of a method using annotations. I would like to be able to use static values, or values from the method arguments.

Rationale
In my Spring Boot application I have several controllers. For each controller, I want to put some values from the method arguments into baggage. I currently do that by wrapping the whole controller like this:

void doSomething(String value1) {
   try (BaggageInScope scope = this.tracer.createBaggageInScope("customer.id", "value1")) {
   // Business logic
   }
}

This approach pollutes my business logic, as every controller is in a try block related to tracing. It would be good if I could so something like:

@Observed
void doSomething(@BaggageInScope((key = "customer.id") String value1) {
   // Business logic
}

Additional context
I've had a look at @ObservationKeyValue which was added under this ticket micrometer-metrics/micrometer#4030. This seems related to this request, but I don't think @ObservationKeyValue puts anything into Baggage scope.

In my Spring Boot application I have written an Aspect to do pretty much the same thing as described above using a custom annotation. It works well. I was thinking something similar would be a good feature in Micrometer generally?

[stuartkemp-e]

Just noticed - I think this is on the wrong project. Should it be on the tracing project?

[jonatan-ivanov]

Thanks for the issue! No worries, I transferred it to Micrometer Tracing.
Have you tried @bclozel's suggestions in spring-projects/spring-boot#48598?

Since you have already written an Aspect for this, I guess it does not help much but you can make this more compact with helpers:

void doSomethingWithBaggage(String id) {
    withBaggage("customer.id", id, this::doSomething);
}

void doSomething(String id) {
    System.out.println(id);
}

<T> void withBaggage(String name, T value, Consumer<T> consumer) {
    try (BaggageInScope ignored = this.tracer.createBaggageInScope(name, String.valueOf(value))) {
        consumer.accept(value);
    }
}

and you can have withBaggage for Runnable, Supplier, etc. the Observation interface has similar helpers but for scopes/observations.

[stuartkemp-e]

Hi @jonatan-ivanov thanks for moving the ticket. Yes, I tried the suggestion in spring-projects/spring-boot#48598. As far as I know, there is no way to put baggage in a method scope using an annotation.

Your example code is nice and concise. I can see it would still mean compromising on readability to add baggage to my services. For example, I would potentially need to gather a few lines of code that exist in the controller into a private method, which I could pass into the Consumer argument. Of course I would also still have the service call wrapped in a 'withBaggage'. I may look into this for my own code, but I think the feature suggestion is still a good one.