Support for system level health check procedures

[heiko-braun]

From @jmesnil

The server runtime could also provide some HealthCheckProcedure that are not application-dependent (e.g. JVM memory, disk space, CPU load). These procedures need to be accessed from a HTTP endpoint not provided by the application (e.g. /health)

[heiko-braun]

@jmesnil Are you willing to draft a design proposal how these procedures could be discovered/registered? I.e. does it require an SPI for that?

[jmesnil]

I'm not sure we need to specify how these "system" health check procedures are discovered and registered. This is internal to the spec implementation (some might use CDI, others have their own injection system).
My initial intent with that issue was to highlight that all health check procedures were not coming from application code and we need to take that into account in our naming convention (e.g for endpoint URL, namespace for health check procedures).

Depending on the outcome of #34, we may simply state in the spec that some namespace (e.g. microprofile.system.*) is reserved for health check procedures provided by the implementation (or runtime).

[antoinesd]

Relation with system should be considered in a broader scope and after 1.1 and introduction of #35

[antoinesd]

This issue is vendor specific and cannot be addressed at specification level