[collectd 6] Migrate the zfs_arc plugin

[octo]

This is a tracking issue for migrating the zfs_arc plugin to collectd 6. Progress is tracked in the collectd 6 project. Technical guidance for migrating plugins to collectd 6 is provided in the V5 to V6 migration guide topic in the wiki.

Build status: Success

The zfs_arc plugin currently compiles, likely thanks to the backwards compatibility layer.

While the backwards compatibility layer allows the plugin to compile, it likely results in suboptimal metric names. If possible, try to find a Prometheus exporter reporting the same metric(s) and use the same naming schema.

Next steps

This is an automatically generated todo list. If one of the following steps doesn't make sense, blame the automation. 🙈

• Assign this issue to yourself.
• Determine whether the zfs_arc plugin should be migrated in the first place. If this is not an obvious "yes", discuss on the mailing list or at the next community call.
• If possible, try to find a Prometheus exporter reporting the same metric(s) and use the same naming schema.
• Update the zfs_arc plugin to produce a metric_family_t and call plugin_dispatch_metric_famliy(). For technical guidance, refer to the V5 to V6 migration guide. You can use the write_log plugin to verify that the produced metrics look as expected.
• Create a pull request referencing this issue, based on the collectd-6.0 branch.