[refactor] Remove the prometheus dependency on pubsub

[aritrbas]

Overview

• This PR removes the Prometheus server's dependency on the pubsub infrastructure.
• The changes establish a direct communication channel between the Felix server and Prometheus for pod lifecycle events.
• The logic for tracking pod interfaces in Prometheus remains identical - only the invocation mechanism is changed from pubsub events to direct method calls.

What Changed

Before:

• Prometheus subscribed to pod events via the pubsub infrastructure, listening to common.PodAdded and common.PodDeleted events on a channel.

After:

• Prometheus is directly owned by the Felix server and receives pod events through explicit method calls.
• Prometheus server is now instantiated directly in NewFelixServer().
• ServePrometheus() is started as a goroutine within ServeFelix() using the tomb lifecycle manager.
• Direct calls to s.prometheusServer.OnPodAdded() and s.prometheusServer.OnPodDeleted() are now made for pod event handling.
• Public methods OnPodAdded() and OnPodDeleted() are added in prometheus.go that directly handle pod lifecycle events.

Event Flow:

• CNI server adds/deletes a pod and publishes PodAdded/PodDeleted event to Felix's event channel.
• Felix's handleFelixServerEvents() processes the event.
• Felix calls workloadAdded()/WorkloadRemoved() to update policy state.
• Felix directly calls s.prometheusServer.OnPodAdded()/s.prometheusServer.OnPodDeleted() to notify Prometheus.
• Prometheus updates its internal pod interface tracking maps.

[sknat]

Thanks for the PR @aritrbas ! lgtm !
let's wait for the parent to be merged before upstreaming this

[sknat]

I think we can cherry pick this part of the refactoring safely on master and merge it before waiting for the rest of the other patches to go in

[sknat]

lgtm ! thanks