Nullpointer exception on metrics generation on cleanup history job

Environment (Required on creation):

camunda ce 7.15.0 ( sprng boot )
linux / windows / docker, does not matter

Description (Required on creation; please attach any relevant screenshots, stacktraces, log files, etc. to the ticket):

• if metrics are disabled by config "metricsEnabled" it does not instantiate a DbMetricsReporter
  org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl#initMetrics ( L2312 )
• and if property "historyCleanupMetricsEnabled" is enabled ( by default ) and not explicitly disabled
• the HistoryCleanupScheduler tries to access the DbMetricsReporter even metrics are disabled and get a nullpoiter exception
  org.camunda.bpm.engine.impl.jobexecutor.historycleanup.HistoryCleanupSchedulerCmd#reportMetrics ( L117)
• in this case it fails to clean the history but also create an entry in ACT_GE_BYTEARRAY without any refference and TTL
• due to the error is thrown many multiple times durng one cleanup job execution, the size of the table grows really fast
• DB entry in ACT_GE_BYTEARRAY with following characteristics: NAME_ = "job.exceptionByteArray" TYPE_ = 3 REV_ = 1
  ID_ BYTES_ and CREATE_TIME_ are filled but the other values are NULL

Steps to reproduce (Required on creation):

• create a default clean engine
• set: camunda.bpm.metrics.enabled= false

Observed Behavior (Required on creation):

every time history scheduler runs it throws a nullpointer exception instead of cleaning the history. and instead of cleaning the history it fills the table more

Expected behavior (Required on creation):

clean history, no nullpointer

Root Cause (Required on prioritization):

please refer to description

Solution Ideas (Optional):

• either extend documentation to set "historyCleanupMetricsEnabled" to false when disable metrics (https://docs.camunda.org/manual/7.15/user-guide/process-engine/metrics/)
• or extend check "org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl#isHistoryCleanupMetricsEnabled" to take general metrics enabled into consideration ( prefered )
• or instantiate dbMetricsReporter when one of the metrics options are enabled

Hints (optional):