feat(router): expose engine statistic metrics

[Noroth]

Motivation and Context

We would like to expose metrics for engine statistics related to subscriptions.

• number of active connections
• number of active subscriptions
• number of active triggers
• count of sent messages from the engine

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]

Dependency Review

✅ No vulnerabilities or OpenSSF Scorecard issues found.

OpenSSF Scorecard

Package	Version	Score	Details

Scanned Files

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-007ac4062634823b561e4f9d738e558c41e185dd

[alepane21]

Just a test for Ludwig

[alepane21]

Just another test for Ludwig

it was just a test

[StarpTech]

Only a few nuts but overall great approach and reuse of WebSocket stats.

[StarpTech]

LGTM