appinfo.xml schema does not allow `openmetrics` element

[SystemKeeper]

OpenMetrics was implemented in #57165, but the schema was not extended to allow openmetrics elements, failing info.xml lints, e.g. https://github.com/nextcloud/spreed/actions/runs/21294040268/job/61295048335?pr=16834

CC @Altahrim