Add "Health as Gauge" how-to documentation

[philwebb]

Micrometer currently includes documentation for exposing Health as Gauge. This should move to Spring Boot so we can keep it up to date.

[snicoll]

@chipkillmar all changes are merged forward, look at the references above your comment and #21091 in particular.

[chipkillmar]

@snicoll thank you! I'm going to delete my previous misleading comment.

[philwebb]

I've raised https://github.com/micrometer-metrics/micrometer/issues/2023