Set explicit permissions in GitHub workflows - `swift-prometheus`

[incertum]

For enhanced security, set explicit permissions for GitHub workflows in this repository. This applies to both the primary "caller" workflows and the reusable "callee" workflows they invoke (which will be updated later in their respective upstream repositories).

This approach aligns with security best practices, as detailed in the following documentation:

• https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#defining-access-for-the-github_token-scopes
• https://openssf.org/blog/2024/08/12/mitigating-attack-vectors-in-github-workflows/