Does anyone know where slack puts the usage information?
For example, how many messages have we used?
How close are we to the dreaded 10,000 message free limit for searching?
How does a particular user (non-admin) backup everything OR better yet automatically post everything to gmail or drive or something else that last longer than 10,000 messages?
You can see the amount of message on the statistics page. https://myslack.slack.com/admin/stats
Usually every Slack user has access to that page (can be restricted to admins only).
To backup your messages you could develop a bot that automatically transfers old messages to an external cloud service and deletes the transferred message in Slack. The Slack API provides the method chat.delete for deleting messages. This approach will also work for private channels if the user that authenticated the bot is a member of that private channel.
Please note that your Slack will continue to function perfectly even after you have reached the 10.000 messages limit, but it will only show the newest 10.000 messages. If you really need to keep all messages you may want to consider either moving up to a paid tier or using a different service.