Add `--output json` to `notify_push:metrics`

[pmarini-nc]

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I just discovered the notify_push:metrics subcommand and thinking about adding the metrics to a Zabbix-based Nextcloud monitoring dashboard that I have developed.

Would it make sense to add the --output json flag to this command? Parsing and transforming into JSON can be done by any scripting language, but as it is widely supported by other occ commands, I thought it made sense to add it here.