Introduced a DynamicValueTag class to determine tag values at runtime

[SimonScholz]

Hi,

In our application we'd love to have common tags for each and every metric, but these common tags might change during runtime. Passing these dynamic common tag values to each and every counter or timer we create is really inconvenient!

This PR is related to this issue: #2223

I was also already able to test this new feature, by adding the following code to the io.micrometer.boot2.samples.components.ServiceLevelObjectiveConfiguration:

    @Bean
    MeterRegistryCustomizer<MeterRegistry> MeterRegistryCommonTagsCustomizer() {
        Tag staticTag = Tag.of("staticTagKey", "static data");
        Tag dynamicTag = Tag.of("dynamicTagKey", () -> "Millis:" + System.currentTimeMillis());
        return registry -> registry.config().commonTags(Arrays.asList(staticTag, dynamicTag));
    }

Then when I started the io.micrometer.boot2.samples.LoggingRegistrySample application together with the MeterRegistryCustomizer mentioned above, I was able to see the different currentTimeMillis in the system out.

Having this feature would save us a lot of effort, because otherwise we would need to pass these dynamic tag values to each and every counter or timer we create in our code.

[SimonScholz]

@jackhammer2k and @jkschneider do you have any objections about this PR?

[jackhammer2k]

I really like the PR and could only find little things in the Javadocs.

[jackhammer2k]

MeterRegistry stores all meters in a map, where Meter.Id is used as key. But this means that Meter.Id.hashcode() or Meter.Id.equals() will be called and must not change over time. But because
Meter.Id.hashcode() calls Tags.hashcode() which calls DynamicValueTag.hashcode(), your implementation must only take the key into account to show the required stable bahavior. However I'm not completely sure if its an issue to not take the value into account in equality checks. Maybe thats the issue jkschneider was talking about.

[SimonScholz]

Let's see what @jkschneider or maybe @izeye think about the equals check.

[jonatan-ivanov]

@SimonScholz Sorry for the big delay. Could you please give use some concrete use-cases with examples? The thing that you are doing in the description seems dangerous since it attaches high-cardinality tags to the meters (Tag.of("dynamicTagKey", () -> "Millis:" + System.currentTimeMillis());).

Also, could you please check if the new withRegistry feature (since 1.12.0) can help? See: #4097

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[SimonScholz]

@jonatan-ivanov Thanks for asking. Meanwhile I left the company, where we've had the need for this feature, but you can find my description of the need here: #2223 (comment)

Basically we used to visualize metrics exposed by a third party (Resilience4J) in our dashboards and wanted to attach certain dynamic tags (e.g., obtain headers from the originating thread local request) to metrics not owned by us.