Review management.observations.long-task-timer.enabled property

It's within the `management.observations` group and has the description "Whether to create a LongTaskTimer for every observation", but it only applies when creating meters from an observation. It also has dashes in the property's prefix which we try to avoid.

Micrometer has an `IgnoredMeters` enum that we may be able to use:

```
management.observations.meters.ignored = long-task-timer
```