perf: Using search engine to compute the total number of records for user metrics

[frascuchon]

Description

Using search engine total instead of DB count to get the total number of records. This improves the performance when running the Argilla server on an HF space with persistent storage enabled.

Type of change

• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)