Add age_in_millis to ILM Explain Response

[gmjehovich]

This PR enhances the _ilm/explain API by adding an age_in_millis field to its response.

Previously, the index's "age" was only reported in a human-readable format (age: "1.97d").

This change introduces the age_in_millis field, providing the index's age as a precise millisecond value (e.g., age_in_millis: 170208000).

This new field is validated with corresponding unit and integration testing

Closes #103659

[elasticsearchmachine]

Hi @gmjehovich, I've created a changelog YAML for you.

[elasticsearchmachine]

Hi @gmjehovich, I've updated the changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[joegallo]

Two tiny comments:

1.) There's a comment block ahead that notes that the array of arguments coming into the parsing constructor ignores a few fields -- since this one will be ignored, we should add it to that comment. Look for this section:

            // a[13] == "age"
            // a[20] == "time_since_index_creation"
            // <--- your new thing would go here

2.) You can DRY the getAge(nowSupplier) call and pull it out as a local final var -- I realize this is pracitically inconsequential, but it allows the world to know I was here. 😉

Other than that, LGTM!

[joegallo]

For the record, I was really flummoxed by how we'd handle disparity between the writer and the reader with a synthetic field like the age_in_millis, but then of course it's just received by the constructor and ignored (which, you know, simple enough). But having a comment to indicate that you know you're ignoring it makes it a little clearer that this wasn't just an oversight.