`LettuceObservationContext` should be constructed with parent `Observation` if exists

[quaff]

Currently LettuceObservationContext.getParentObservation() rely on micrometer default implementation, which get current observation from ThreadLocal, but lettuce is executed in its own thread other than parent observation's thread, which lead to LettuceObservationContext.getParentObservation() be null even it called within a parent observation scope, LettuceObservationContext.setParentObservation() should be called before observation created, then we could use LettuceObservationContext.getParentObservation() in ObservationPredicate, see also micrometer-metrics/micrometer#3867.

https://github.com/micrometer-metrics/micrometer/blob/main/micrometer-observation/src/main/java/io/micrometer/observation/SimpleObservationRegistry.java#L30-L47

[quaff]

Superseded by #2592