In-memory collector: merge reports after upgrade

Upgrading reports individually reduces the amount of upgrades, since some reports might have been upgraded in previously.

app/collector.go

@@ -152,7 +152,13 @@ func (c *collector) Report(_ context.Context, timestamp time.Time) (report.Repor
 	c.clean()
 	c.quantise()
 
-	rpt := c.merger.Merge(c.reports).Upgrade()
+	upgradedReports := make([]report.Report, 0, len(c.reports))
+	for _, r := range c.reports {
+		upgradedReports = append(upgradedReports, r.Upgrade())
+	}
+	c.reports = upgradedReports
+
+	rpt := c.merger.Merge(c.reports)
 	c.cached = &rpt
 	return rpt, nil
 }