Don't count a per-view counter going down as a restart of named

After a reconfig that changed named's views, the cache and resolver
counters (summed over views) went down, which was taken for a restart:
every counter then counted its total since named started for one minute.
Only a new boot time means a restart now; a counter that goes down
counts 0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/stats/poll_test.go

@@ -47,9 +47,15 @@ func TestFlattenAndDelta(t *testing.T) {
 	if d := Delta(s2, s3); d["queries"] != 3 || d["qtype:A"] != 3 {
 		t.Errorf("restart delta = %v", d)
 	}
-	// A counter going down also means a restart.
-	s4 := &Snapshot{BootTime: s2.BootTime, Values: map[string]int64{"queries": 2}}
-	if d := Delta(s2, s4); d["queries"] != 2 {
-		t.Errorf("reset delta = %v", d)
+	// A counter going down without a restart (views changed in a
+	// reconfig) counts 0, and doesn't make the others count from zero.
+	s4 := &Snapshot{BootTime: s2.BootTime, Values: map[string]int64{}}
+	for k, v := range s2.Values {
+		s4.Values[k] = v
+	}
+	s4.Values["queries"] += 4
+	s4.Values["cache:hits"] = 1 // was 4: a view's cache counters are gone
+	if d := Delta(s2, s4); d["queries"] != 4 || d["cache:hits"] != 0 || len(d) != 1 {
+		t.Errorf("delta with a counter going down = %v", d)
 	}
 }