Fix BenchmarkScrapeLoopAppendOM

OpenMetrics requires EOF comment at the end of metrics body, but the makeTestMetrics() function doesn't append it.
This means this benchmark tests a response with errors but I don't think that was the intention.

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

scrape/scrape_test.go

@@ -1068,6 +1068,7 @@ func makeTestMetrics(n int) []byte {
 		fmt.Fprintf(&sb, "# HELP metric_a help text\n")
 		fmt.Fprintf(&sb, "metric_a{foo=\"%d\",bar=\"%d\"} 1\n", i, i*100)
 	}
+	fmt.Fprintf(&sb, "# EOF\n")
 	return sb.Bytes()
 }