I've wanted a badge that shows the instance uptime and couldn't find any, so I created one. It's available at https://uptime.lemmings.world/{your-domain}.svg and gets its data from fediverse.observer.

Examples:

• Lemmings.world: - https://uptime.lemmings.world/lemmings.world.svg
• Lemmy.world: - https://uptime.lemmings.world/lemmy.world.svg
• Lemmy.ml: - https://uptime.lemmings.world/lemmy.ml.svg
• discuss.tchncs.de: - https://uptime.lemmings.world/discuss.tchncs.de.svg (congrats on that uptime, by the way!)
• and so on

The documentation is available at https://uptime.lemmings.world. Feel free to use it for your instance if you wish to.