logging: Add subsystem to log API (#19002)

Create new code paths for multiple subsystems in the code. This will
make maintaing this easier later.

Also introduce bugLogIf() for errors that should not happen in the first
place.

internal/logger/audit.go

@@ -36,7 +36,7 @@ const contextAuditKey = contextKeyType("audit-entry")
 // SetAuditEntry sets Audit info in the context.
 func SetAuditEntry(ctx context.Context, audit *audit.Entry) context.Context {
 	if ctx == nil {
-		LogIf(context.Background(), fmt.Errorf("context is nil"))
+		LogIf(context.Background(), "audit", fmt.Errorf("context is nil"))
 		return nil
 	}
 	return context.WithValue(ctx, contextAuditKey, audit)
@@ -144,7 +144,7 @@ func AuditLog(ctx context.Context, w http.ResponseWriter, r *http.Request, reqCl
 	// Send audit logs only to http targets.
 	for _, t := range auditTgts {
 		if err := t.Send(ctx, entry); err != nil {
-			LogOnceIf(ctx, fmt.Errorf("Unable to send an audit event to the target `%v`: %v", t, err), "send-audit-event-failure")
+			LogOnceIf(ctx, "logging", fmt.Errorf("Unable to send an audit event to the target `%v`: %v", t, err), "send-audit-event-failure")
 		}
 	}
 }