MINOR: stats: add check and agent's health values in the output

MINOR: stats: add check and agent's health values in the output

This adds the following fields :
- check_rise [...S]: server's "rise" parameter used by checks
- check_fall [...S]: server's "fall" parameter used by checks
- check_health [...S]: server's health check value between 0 and rise+fall-1
- agent_rise [...S]: agent's "rise" parameter, normally 1
- agent_fall [...S]: agent's "fall" parameter, normally 1
- agent_health [...S]: agent's health parameter, between 0 and rise+fall-1