Enabled: always must skip two levels

klog does stack unwinding in `LogSink.Enabled` to implement per-source code
verbosity thresholds (`-vmodule`). This worked for `logger.Info` and
`logger.Error` because the code was written such that it matches how logr is
implemented (Logger.Info -> Logger.Enabled -> LogSink.Enabled).

It did not work for direct calls (`if logger.Enabled`) because then the call
chain is `Logger.Enabled -> LogSink.Enabled`. That callchain is less common, so
to fix this problem the callchains get arranges so that all calls go through
one additional intermediate level.

This is unnecessary extra work, but it is necessary to avoid breaking the more
common normal logging calls with klog and -vmodule.

logr.go

@@ -258,7 +258,15 @@ type Logger struct {
 // Enabled tests whether this Logger is enabled.  For example, commandline
 // flags might be used to set the logging verbosity and disable some info logs.
 func (l Logger) Enabled() bool {
-	return l.sink != nil && l.sink.Enabled(l.level)
+	return l.sink != nil && l.enabled(l.level)
+}
+
+// enabled ensures that Enabled and Info both call LogSink.Enabled with one
+// intermediate stack frame. This wouldn't be necessary if Info had called
+// LogSink.Enabled directly in logr v1.0.0, but it didn't and now that cannot
+// be changed anymore because implementations depend on this behavior.
+func (l Logger) enabled() bool {
+	return l.sink.Enabled(l.level)
 }
 
 // Info logs a non-error message with the given key/value pairs as context.
@@ -271,7 +279,7 @@ func (l Logger) Info(msg string, keysAndValues ...any) {
 	if l.sink == nil {
 		return
 	}
-	if l.Enabled() {
+	if l.enabled() {
 		if withHelper, ok := l.sink.(CallStackHelperLogSink); ok {
 			withHelper.GetCallStackHelper()()
 		}
@@ -447,9 +455,13 @@ func NewContext(ctx context.Context, logger Logger) context.Context {
 // LogSinks might want to know.
 type RuntimeInfo struct {
 	// CallDepth is the number of call frames the logr library adds between the
-	// end-user and the LogSink.  LogSink implementations which choose to print
+	// end-user and the LogSink. LogSink implementations which choose to print
 	// the original logging site (e.g. file & line) should climb this many
 	// additional frames to find it.
+	//
+	// The CallDepth is given for LogSink.Info and LogSink.Error such that 1 means
+	// "skip one intermediate level". For historic reasons, LogSink.Enabled must
+	// skip one additional level compared to those two calls.
 	CallDepth int
 }