Use Tag#compareTo to detect duplicates

[datibbaw]

Addresses #1432 by using the compareTo() method from the Tag interface, defined here

default int compareTo(Tag o) {
    return getKey().compareTo(o.getKey());
}

The above is already marked as overridable and implements the existing dedup logic almost exactly:

for (int i = 0; i < n - 1; i++)
    if (!tags[i].getKey().equals(tags[i + 1].getKey()))
        tags[j++] = tags[i];

In this way, developers can extend ImmutableTag, which implements Tag, with their own class to override the compareTo() method like so.

@Override
public int compareTo(Tag o) {
  return Comparator.comparing(Tag::getKey).thenComparing(Tag::getValue).compare(this, o);
}

To be clear, this isn't meant to be made available by default; because of its limited applicability, developers take on the risk of passing the appropriate values to the corresponding backend.

[shakuzen]

Thank you for the pull request.
As mentioned in the linked issue, I think backends other than Datadog do not support what you're wanting to do. So allowing something like this in micrometer-core is likely to cause problems unless you're only publishing metrics to Datadog. So I'm not sure yet that this is the best approach.

[datibbaw]

The idea behind this approach is that existing code will continue to work in the same way, i.e. tags are deduplicated based on their keys; developers are required to override the Tag#compareTo() method with a class of their own, as a means to signal that they understand the consequences of their actions.

But maybe I've missed something; this is a big codebase, and I don't assume to know a whole lot about it other than attempting to scratch an itch for our own projects.

[datibbaw]

@shakuzen Could you have another look at this? The code change itself doesn't do anything new, rather it uses the existing Tag interface to accomplish the same functionality while opening the implementation up for extension where necessary. I've added an example of this in the tests section.

[datibbaw]

@shakuzen Hoping to get an answer to my earlier question; even though we use only Datadog in our organisation, it's the abstraction of metrics that we like to keep using.

[shakuzen]

Sorry for the lack of response. I'll take a closer look at this again next week.