[Discuss] Should we allow users to disable collection of metrics

@L4rS6 discovered an issue with our metric collection; it doesn't work when procfs is not mounted.
For security reason users can disable the access of procfs, this restriction will make logstash crash at startup.

For me, its a bug and we should resolve #6296, but let's open the discussion should we allow users to be able to disable the metric collection? Because right now 5.0 is not usable in that scenario and there is no workaround until we fixed the issue.

Start with issue #6296 and the procfs-related metric collection behavior described here. Determine how disabling collection should affect startup and define acceptance criteria for users who cannot access procfs; the issue currently provides no named files or tests.

Written by the indexing model from the issue text.