Add metrics for `push-notification-server`

[mendelskiv93]

Problem

The push-notification-server currently doesn't expose any metrics, making it difficult to monitor since there is no visibility into node operation (registrations, push attempts, errors, etc.).

Without these metrics, we cannot determine if push notifications are failing at the status-go level (e.g., failing to send requests to gorush) even when gorush appears healthy.

Request:

Add metrics exposure to the push-notification-server to enable proper monitoring. While we can monitor gorush performance through its metrics, we have no visibility into the status-go push notification server itself.

Suggested Implementation:

Add metrics endpoint (e.g., /metrics for Prometheus)

• TODO

Ref.:

• https://github.com/status-im/infra-push-notify/issues/11