The 'average time to close' for cases calculation needs api support to distinguish between zero and no-closed-cases in the UI

[EricDavisX]

Kibana version:
8.3.0 BC1 - cloud deploy
which is Kibana commit: ad53233
from Date: Wed Jun 1 06:02:11 2022 -0500

Browser version:
Chrome on mac Version 102.0.5005.61 (Official Build) (x86_64)

Original install method (e.g. download page, yum, from source, etc.):
cloud staging

Describe the bug:
Discussed with Christos N on Cases team. The value for that calc is rounded down to nearest whole second. If a cases are closed immediately when created (based on cases connectors configuration setting to) the KPI value can show as a dash, indicating no value is known, when it is more accurately represented with a near-zero ~0.

that api call in question is:
/api/cases/metrics?owner=cases&features=mttr
it is currently returning:
{"mttr":0}
and this is correct. however when there are no cases or no closed cases, we would like it to return null, to better distinguish the state so the UI can adjust the display to be more accurate and friendly.

Note: The plan is for the UI will show a - if there are no closed cases and to show a ~0 if the number is reported as zero.

Steps to reproduce:

1. in a fresh Kibana, call the api above
   /api/cases/metrics?owner=cases&features=mttr

it should return with null value for mttr.

2. if you then add a case and close it, the api should report the number back, including the zero case (where cases are immediately closed due to case preference setting)

** other**
Feature is introduced in 8.3.0 - so it is new.

This is somewhat of a special case, but maybe it would be seen in larger scale usage too? I'm not sure. I do think it would benefit for trial users or new stack users. For that alone it may be helpful enough and generally less confusing to implement the more user friendly, and accurate-at-low-case-volume api change.

** screenshots**
after noting it is a special case, it seems possible / likely to happen... see this example and how it looks strange not to have a calculation available (this was a choice because the UI couldn't distinguish between a true zero and no-closed cases)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[EricDavisX]

tested and validated in 8.3 BC6 and a test requirement was added to the QA list