MM-22786 enterprise metrics for logging (#15313)

Adds metrics for logging engine which are collected by Prometheus.
- current queue level(s)
- rate of logging records emitted
- rate of logging errors

mlog/logr.go

@@ -28,7 +28,7 @@ type LogLevel struct {
 }
 
 type LogTarget struct {
-	Type         string // one of "console", "file", "tcp", "syslog".
+	Type         string // one of "console", "file", "tcp", "syslog", "none".
 	Format       string // one of "json", "plain"
 	Levels       []LogLevel
 	Options      json.RawMessage
@@ -38,7 +38,7 @@ type LogTarget struct {
 type LogTargetCfg map[string]*LogTarget
 type LogrCleanup func() error
 
-func newLogr(targets LogTargetCfg) (*logr.Logger, error) {
+func newLogr() *logr.Logger {
 	lgr := &logr.Logr{}
 	lgr.OnExit = func(int) {}
 	lgr.OnPanic = func(interface{}) {}
@@ -46,12 +46,12 @@ func newLogr(targets LogTargetCfg) (*logr.Logger, error) {
 	lgr.OnQueueFull = onQueueFull
 	lgr.OnTargetQueueFull = onTargetQueueFull
 
-	err := logrAddTargets(lgr, targets)
 	logger := lgr.NewLogger()
-	return &logger, err
+	return &logger
 }
 
-func logrAddTargets(lgr *logr.Logr, targets LogTargetCfg) error {
+func logrAddTargets(logger *logr.Logger, targets LogTargetCfg) error {
+	lgr := logger.Logr()
 	var errs error
 	for name, t := range targets {
 		target, err := NewLogrTarget(name, t)
@@ -59,7 +59,9 @@ func logrAddTargets(lgr *logr.Logr, targets LogTargetCfg) error {
 			errs = multierror.Append(err)
 			continue
 		}
-		lgr.AddTarget(target)
+		if target != nil {
+			lgr.AddTarget(target)
+		}
 	}
 	return errs
 }
@@ -90,6 +92,8 @@ func NewLogrTarget(name string, t *LogTarget) (logr.Target, error) {
 		return newSyslogTarget(name, t, filter, formatter)
 	case "tcp":
 		return newTCPTarget(name, t, filter, formatter)
+	case "none":
+		return nil, nil
 	}
 	return nil, fmt.Errorf("invalid type '%s' for target %s", t.Type, name)
 }
@@ -201,6 +205,10 @@ func checkFileWritable(filename string) error {
 }
 
 func isLevelEnabled(logger *logr.Logger, level logr.Level) bool {
+	if logger == nil || logger.Logr() == nil {
+		return false
+	}
+
 	status := logger.Logr().IsLevelEnabled(level)
 	return status.Enabled
 }