feat: intial prom endpoint

[GoliathLabs]

Here's a possible fix for #918.

Hasn't been tested yet.

Closes #918

[mergeable]

Thanks for creating a pull request! A maintainer will review your changes shortly. Please don't be discouraged if it takes a while.

[GoliathLabs]

Was able to test it on my end @T0biii @DasSkelett

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[T0biii]

maybe @Operating3336 can test this pr as well?

[GoliathLabs]

@Operating3336 you can test this out by using the docker pull ghcr.io/freifunkmuc/wg-access-server:prometheus tag. Feedback would be much appreciated :)

[GoliathLabs]

@copilot I've got the following task: Switch default metadata exposure for Prometheus endpoint. Currently, the VPN server Prometheus endpoint uses DisableMetadata by default. Exposing user stats by default might not be fully privacy-compliant.

Replace DisableMetadata with EnableMetadata in the implementation. Change the logic to honor that. Update the docs as well.

[Copilot]

@GoliathLabs I've opened a new pull request, #1051, to work on those changes. Once the pull request is ready, I'll request review from you.