feat: Implement Monitoring Stage in Dockerfile

[gustavovalverde]

Motivation

The main Dockerfile contains a TODO for a monitoring stage. Implementing this will create a standardized, optimized image for our performance observation environment, separating it from development and release builds.

Specifications

• Add a new monitoring stage to docker/Dockerfile.
• This stage should build upon the runtime stage.
• It must build zebrad with the prometheus feature enabled.
• It should include any agents or configurations needed for log forwarding to GCP, if applicable.
• Update the deployment process to use this monitoring target for the observability environment.