only amend iptables-rules prom gauge when a chain is inc/dec-ref'd

[aaaaaaaalex]

Description

Related issues/PRs

Todos

• [ ] Tests
• [ ] Documentation
• [ ] Release note

Release Note

The `felix_iptables_rules` Prometheus metric now only counts rules which reference an iptables chain, rather than counting all candidate rules (even when they may not reference anything or be committed to the kernel).

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