Define health metrics group

[lambdanis]

Depends on https://github.com/cilium/tetragon/pull/2606 (first commit)

There are two things happening here:

1. Refactored custom collectors to use new helpers from pkg/metrics (commits 2-4). The main win here is simplifying how custom metrics are included in reference docs - we don't need to define a separate type and register it separately just for generating docs.
2. Defined a health metrics group (last commit). The idea is that in the future this group will have constrained cardinality and will be enabled by default (in contrast to another group with potentially high cardinality "debug" metrics). This commit only refactors the existing metrics initialization code to use the new framework. The health metrics group contains all metrics that were documented in the "health metrics" section, but in the future some of them will likely be moved to another group.