Unrestricted Health check should be able to be cached

[ericdahl]

If endpoints.health.sensitive is set to false, then the caching feature of the Health endpoint is bypassed due to this check in HealthMvcEndpoint:

if (isCacheStale(accessTime) || isSecure(principal) || isUnrestricted()) {
    this.lastAccess = accessTime;
    this.cached = this.delegate.invoke();
}

It would be nice to still be able to have the caching functionality even if the health endpoint is unrestricted. For example, if I have an intensive health check which is unrestricted, but running on a local IP or otherwise protected, I may still want to have the value be cached so that an overly zealous health check doesn't cause the service to degrade.

[wilkinsona]

I have a fix for this issue here.

@philwebb I'd welcome your thoughts on the fix. Specifically, I'm wondering if we might also want a property to force caching for secure access. If we do, or we think we might in the future, then a single, more generally named property could be preferable to one that controls caching for unrestricted access and one that controls caching for secure access. endpoints.health.always-cache, perhaps?

[philwebb]

@wilkinsona We could consider just always caching results. The current logic is a little complex and I'm wondering if the time-to-live property alone is enough for most people. The docs already imply that caching occurs regardless of how the endpoint is accessed or if it is secured.

[wilkinsona]

@philwebb Sold. The current logic made my head hurt a bit when I was working on this fix; simplifying it definitely sounds good to me.