AthenaMonitoringKernel: always set kAlwaysCreate for lumiblock histograms online

For the correct functioning of the Gatherer, lumiblock histograms should always be created with the kAlwaysCreate option to ensure they are re-booked on each LB change.

Relates to ADHI-4947.

cc @salnikov

P.S.: Should have really done this a long time ago. I was reminded by @stelzer in !64147 (merged).

Control/AthenaMonitoringKernel/python/GenericMonitoringTool.py

@@ -484,6 +484,11 @@ def defineHistogram(flags, varname, type='TH1F', path=None,
         assert type not in ['TEfficiency', 'TTree', 'TGraph'],'only default merge defined for non-histogram objects'
         settings['merge'] = merge
 
+    # LB histograms always need to be published online (ADHI-4947)
+    if settings['kLBNHistoryDepth']>0 and _isOnline(flags):
+        settings['kAlwaysCreate'] = True
+        log.debug('Setting kAlwaysCreate for lumiblock histogram "%s"', varname)
+
     # Check that kLBNHistoryDepth and kLive are both non-negative
     assert settings['kLBNHistoryDepth']>=0, f'Histogram "{alias}" has invalid kLBNHistoryDepth.'
     assert settings['kLive']>=0, f'Histogram "{alias}" has invalid kLive.'