Mattermost: Add grafana dashboard

Package Merge Request

Package Changes

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/blob/7.1.2-bb.1/CHANGELOG.md

Package MR

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/96

For Issue

Closes https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/1214