Allow multiple MeterTag annotations for multiple tags from same target

[gwelican]

Please describe the feature request.
The '@MeterTag' feature lets you add dynamic tags, taking a value from a parameter. Currently, you can only extract one key/value pair because annotation can't be repeated and I did not see a way to define a list of key/value pairs.
It would be helpful to allow for extracting multiple key-value pairs.

Rationale
For APIs with numerous fields, it would be beneficial to create multiple tags for improved filtering.

Additional context

[marcingrzejszczak]

Maybe I'm missing sth, can you provide an example of what doesn't work currently?

[gwelican]

hey @marcingrzejszczak, thanks for taking a look. Suppose we have the following code:

public ChargeResponse chargeInstrument(ChargeInstrumentRequest chargeInstrumentRequest) {
      }

public class ChargeInstrumentRequest {
 String currency; // EUR, USD etc
 String instrumentType; // CARD, PAYPAL etc
}

I can use @MeterTag to extract 1 field:

      @MeterTag(key = "instrument_type", expression = "instrumentType")

If I also want to use the currency as a tag, I don't see a way to do it with @MeterTag, I cannot repeat @MeterTag and @MeterTag does not accept a list of keyvaluepairs, I could implement my custom extractor, but would be nice to use spel with @MeterTag.

I am still on spring boot 2.x, planning to upgrade soon, my apologies if this was solved in 3.x, but from the micrometer documentation, it seemed like this is still going to be a problem in 3.x

[marcingrzejszczak]

Do I understand correctly that you would like to use a single method parameter as 2 tags? In other words annotate the currency parameter twice with different values?

[gwelican]

Correct, I'd like to do

public ChargeResponse chargeInstrument(@MeterTag(key = "instrument_type", expression = "instrumentType") @MeterTag(key = "currency", expression = "currency") ChargeInstrumentRequest chargeInstrumentRequest) {
}

Or if the annotation cannot be repeated(it is just an example). Providing a key-value pair is probably more readable, where the key is the metric key and the value is the spel that resolves the value from the input parameter.

[smaxx]

it seems that the only missing thing is the declaration of @Repeatable(MeterTags.class) where MeterTags is just:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.PARAMETER})
@Documented
public @interface MeterTags {
    MeterTag[] value();
}

[marcingrzejszczak]

Care for a PR?

[smaxx]

Sure #5055