prometheus_sysctl_exporter: fix metric aliasing

When exporting sysctls to Prometheus, the exporter replaces "." with
"_". This caused several metrics to alias, confusing the Prometheus
server. Fix it by:

• Renaming the "tcp_log_bucket" UMA zone to "tcp_log_id_bucket". Also, rename "tcp_log_node" to "tcp_log_id_node" for consistency.

• Not exporting sysctls with "(LEGACY)" in the description. That is used by ZFS sysctls that have been replaced by others, many of which alias to the same Prometheus metric name (like "vfs.zfs.arc_max" and "vfs.zfs.arc.max").

PR: 259607
Reported by: delphij
Sponsored by: Axcient
Reviewed by: delphij,rew,thj
Differential Revision: https://reviews.freebsd.org/D34952

(cherry picked from commit 8c47d8f53854825d8e8591ccd06e32b2c798f81c)