Show configured metrics retention (Refs #141)

src/lib/server/fleet/metrics.ts

@@ -507,6 +507,7 @@ export function readMetrics(input: MetricsDays | MetricsQuery, now = Date.now())
     };
     return {
         days,
+        metricsKeepDays: fleetSettings().metricsKeepDays,
         since,
         now,
         ...metadata,