Updating inference services API response

[ymao1]

❗❗This should not be merged until elastic/kibana#203363 is released to serverless

Resolves https://github.com/elastic/ml-team/issues/1428

Summary

Updates the GET _inference/_services API to follow the new agreed upon response format.

For the new updatable field, I followed the guidance from API docs which says you can modify task_settings, secrets (within service_settings), or num_allocations, depending on the specific endpoint service and task_type you’ve created, so the only fields with updatable: true are secret settings field and num_allocations.

[elasticsearchmachine]

Hi @ymao1, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[davidkyle]

LGTM, I have a couple of suggestions

The output contains lots of entries like "default_value": null,. In SettingsConfiguration#toXContent() please can you wrap writing default_value in a null check.

The service configurations are not sorted as order they are written depends on the iteration order of a map. Sorting the individual configs by service would make the output more readable.

[ymao1]

The output contains lots of entries like "default_value": null,. In SettingsConfiguration#toXContent() please can you wrap writing default_value in a null check.

Updated in 8e1562d

The service configurations are not sorted as order they are written depends on the iteration order of a map. Sorting the individual configs by service would make the output more readable.

Updated in bc1922d

[YulNaumenko]

LGTM! Tested locally, works as expected.