Service labels not included in Prometheus service discovery response

## Problem

The Prometheus service discovery endpoint was not including custom service labels (such as `route_id`, `endpoint_id`, `model_service_name`, `runtime_variant`) in the response.

## Root Cause

When building the Prometheus service discovery response in `manager/server.py`, only hardcoded labels (`service_id`, `service_group`, `display_name`) were included. The `service.labels` dictionary containing custom labels from `ModelServiceMetadata` was not being spread into the response.

## Fix

Added `**service.labels` spread to include all custom service labels in the Prometheus service discovery response.

JIRA Issue: BA-3471