Construct LettuceObservationContext with parent observation

[quaff]

After this commit, LettuceObservationContext.setParentObservation() is called right after Context rather than Observation created, then Context.getParentObservation() could be used in ObservationPredicate to determine whether Observation should be created, for example, we can filter out parentless lettuce observations like this:

	@Bean
	ObservationPredicate noParentlessLettuceObservations() {
		return (name, context) -> {
			if (context instanceof LettuceObservationContext) {
				return context.getParentObservation() != null;
			}
			return true;
		};
	}

Fix GH-2591

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[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.