monitoring: stats.json freshness + display fields outside-in

scripts/taler-exchange/landing-stats-exchange.sh

@@ -238,6 +238,7 @@ fi
 
 GEN=$(now_iso)
 HUMAN=$(now_human)
+GEN_UNIX=$(date +%s)
 num_or_null() { case "${1:-}" in ''|null) echo null ;; *) echo "$1" ;; esac; }
 
 MEM_JSON='"container_rss_human": "—"'
@@ -257,6 +258,7 @@ cat >"$TMP" <<EOF
   "timezone": $(json_str "$TZ"),
   "generated_at": $(json_str "$GEN"),
   "generated_at_human": $(json_str "$HUMAN"),
+  "generated_at_unix": ${GEN_UNIX},
   "reserves": ${RESERVES:-0},
   "wire_in_count": ${RESERVES_IN:-0},
   "wire_in_amount": $(json_str "$WIRE_IN_AMT"),