Extend logging for Mirror jobs

What does this MR do and why?

Contributes to #364799 (closed)

Log status of the current load balancing session.

Screenshots or screen recordings

2022-06-15T13:59:13.084Z: Mirror job scheduled for root/test-project with job ID 44aa95d5b2bec087f15d3972 (primary: true).
2022-06-15T13:59:13.480Z: Mirror update for root/test-project started. Waiting duration: 0
2022-06-15T13:59:14.480Z: Mirror update for root/test-project successfully finished. Update duration: 1

# From log/sidekiq.log
...
"wal_location_source":"primary"
...

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.