Martin Milata 87f87fb3e9 nixos/prometheus-snmp-exporter: update log options
The allowed values have changed in bd3319d28c43580c74839a10dca5d348cc3e0b29.

0.15:
      --log.level="info"        Only log messages with the given severity or above. Valid levels: [debug, info, warn, error, fatal]
      --log.format="logger:stderr"
                                Set the log target and format. Example: "logger:syslog?appname=bob&local=7" or "logger:stdout?json=true"

0.17:
      --log.level=info          Only log messages with the given severity or above. One of: [debug, info, warn, error]
      --log.format=logfmt       Output format of log messages. One of: [logfmt, json]
2020-03-06 01:43:20 +01:00
..
2020-03-05 14:03:27 +01:00
2020-03-05 18:00:28 +00:00

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