Added literal string unions to OpenApiDocs, #PG-5012

[lachiebol]

Description

AbTesting has values you can test in the $period param (getMetricsOverview)

Issue No

Steps to Replicate the Issue

Checklist

• [✔] Tested locally or on demo2/demo3?
• [✔] New test case added/updated?
• [NA] Are all newly added texts included via translation?
• [NA] Are text sanitized properly? (Eg use of v-text v/s v-html for vue)
• [✔] Version bumped?
• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules