Variable HEALTH_METRICS_FIRST_UPLOAD_MSConst

The Pigeon logger's health counters, /pigeon_health_metrics, as the app sends them (the facts are in this package's spec/):

• the first upload goes 15 s after the process started, then every 240 s;
• only in a process that started with a session;
• it carries the counters since the last delivered upload, so the first upload of a process carries the counts of the earlier process's tail.