[backend] improve file indexing metrics performance (#7530)

[SouadHadjiat]

Proposed changes

• use aggregation to count files by mime type

Related issues

• #7530

performance issue not reproduced locally => need to test queries directly on DB (with platform team)

Checklist

• [x] I consider the submitted work as finished
• [x] I tested the code for its functionality
• [ ] I wrote test cases for the relevant uses case (coverage and e2e)
• [ ] I added/update the relevant documentation (either on github or on notion)
• [ ] Where necessary I refactored code to improve the overall quality