Mattermost is grouping calls in the /metrics endpoint by id and reports that id in the response. Since this id is the channelID, the public /metrics endpoint is revealing channelIDs.
{
"cpes": [
"cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*"
],
"severity": "Medium"
}