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

[wilkinsona]

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

[snicoll]

Turns out this was mostly done, we only need a proper deprecation of the old property + code cleanup.