feat: add leader instance identification with operational metadata

- Store instance metadata (port, PID, hostname, instance_id) in Redis as JSON
- Add is_leader boolean field to /health endpoint for monitoring
- Make /health endpoint async to properly check Redis leadership status
- Update tests to verify JSON metadata format and async health checks
- Maintain backward compatibility with legacy UUID-only format
- Remove unused is_leader_sync() method to avoid event loop conflicts

Closes #3838

Signed-off-by: Bogdan-Marius-Catanus <bogdan-marius.catanus@ibm.com>

Author: Bogdan-Marius-Catanus