fix: use gauge for server.uptime metric

[StarpTech]

Motivation and Context

This PR change the server uptime metric from type counter to gauge. Counter was not correct because we are not interested in the aggregation but in the last value.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-804bae228417cb3989adef55b3ff695a8086d0be

[Noroth]

LGTM