Make UnregisterSet() less error-prone to use

Previously it was expected that the user calls s.UnregisterAllMetrics() after calling UnregisterSet(s).
This led to many subtle memory leak bugs like https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6247 .

Solve this issue by adding `destroySet bool` arg to UnregisterSet(), so it automatically calls
s.UnregisterAllMetrics() if destroySet=true.

This changes UnregisterSet() function signature, so users need to update UnregisterSet() calls across their code bases
after upgrading to the new version of github.com/VictoriaMetrics/metrics package. This is OK, since this allows
fixing subtle memory leak bugs like https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6247 .

metrics.go

@@ -55,11 +55,16 @@ func RegisterSet(s *Set) {
 
 // UnregisterSet stops exporting metrics for the given s via global WritePrometheus() call.
 //
-// Call s.UnregisterAllMetrics() after unregistering s if it is no longer used.
-func UnregisterSet(s *Set) {
+// If destroySet is set to true, then s.UnregisterAllMetrics() is called on s after unregistering it,
+// so s becomes destroyed. Otherwise the s can be registered again in the set by passing it to RegisterSet().
+func UnregisterSet(s *Set, destroySet bool) {
 	registeredSetsLock.Lock()
 	delete(registeredSets, s)
 	registeredSetsLock.Unlock()
+
+	if destroySet {
+		s.UnregisterAllMetrics()
+	}
 }
 
 // RegisterMetricsWriter registers writeMetrics callback for including metrics in the output generated by WritePrometheus.

metrics_test.go

@@ -148,7 +148,7 @@ func TestRegisterUnregisterSet(t *testing.T) {
 		t.Fatalf("missing %q in\n%s", expectedLine, data)
 	}
 
-	UnregisterSet(s)
+	UnregisterSet(s, true)
 	bb.Reset()
 	WritePrometheus(&bb, false)
 	data = bb.String()