- comment, clean up, optimize code

- drop deprecated pmconfig.py methods

- drop obsolete metricsets from config files

- update sar/iostat/pidstat metricsets to match latest sysstat
  - last check: Aug '25
  - for :sar-r change later:
    - https://github.com/sysstat/sysstat/commit/6f34da35732d05a27fd2723ff9ef0511733c2757
    - https://github.com/sysstat/sysstat/commit/99eacd7e203a29867c9245d510ce931e8ce1341b
  - for :pidstat consider these if needed:
    - https://github.com/sysstat/sysstat/commit/7f9a1d45467ba5b1cefcbd7b1b7369373e272867
    - https://github.com/sysstat/sysstat/commit/378d097d7c5af3b4935d04a9f12f1dbd85b4b854

- verify metric labels printing (-m)
  - test instance vs metric labels
- enhanced metric labels support [-M]
  - report metrics based on matched labels (pmrep -M foo:env=prod)
- prom2json like output
  - https://github.com/prometheus/prom2json
- consider metric labels support in pmrep.conf

- change default precision to 2 (?)
- generalized zabbix_interval (write to target in batches) (?)

- check and/or clean "# Instance(s)" checks in pmconfig.py (??)
- there was some issue with derived metrics, probably best not to
  check indom but PM_IN_NULL to make sure everything needed for derived
  is being fetched (???) - see https://groups.io/g/pcp/message/21806

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