Check if public_metrics are enabled

[travisdowns]

Check if disable_public_metrics is set and in this case do not register the available_memory related metrics on the public metrics endpoint.

Cover letter

Without this change, the metrics show up on the public endpoint even when that endpoint is disabled.

Backport Required

• [ ] not a bug fix
• [ ] issue does not exist in previous branches
• [ ] papercut/not impactful enough to backport
• [x] v22.3.x
• [ ] v22.2.x
• [ ] v22.1.x

Release notes

• none

Improvements

Metrics do not show up in public_metrics when endpoint is disabled.