Fix null redisClient during validation, add more logs

[s01cy]

I've noticed a bug in RedisExporter - it is trying to do this:

During exporter validation, Zeebe tries to test the lifecycle methods of the exporter
The validation process was calling close() without first calling open()
This led to redisClient being null when close() was called

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[VonDerBeck]

@s01cy
Thanks a lot for this! Even small changes making the exporter more stable are more than welcome. 👍

Can you do me a favour and fix the failing RedisLateStartupTest? It fails due to your changed log output - the test uses log messages as an indicator e.g. in order to detect when the initial connection to Redis has failed etc.

Gunnar

[VonDerBeck]

This change is ok. But needs to be considered in RedisLateStartupTest line 72/73 where the test waits until the exporter failed to connect to Redis. This ensures that the later startup of Redis orchestrated by the test will be really after this point.