Rename label-related variables for homeservers

We'd be adding integration with an internal Traefik entrypoint
(`matrix_playbook_internal_matrix_client_api_traefik_entrypoint`),
so renaming helps disambiguate things.

There's no need for deperecation tasks, because the old names
have only been part of this `bye-bye-nginx-proxy` branch and not used by
anyone publicly.

roles/custom/matrix-synapse/templates/synapse/prometheus/external_prometheus.yml.example.j2

@@ -27,7 +27,7 @@ scrape_configs:
   - job_name: '{{ worker.name }}'
     metrics_path: /metrics/synapse/worker/{{ worker.id }}
     scheme: https
-{% if matrix_synapse_worker_container_labels_metrics_middleware_basic_auth_enabled|default(true) %}
+{% if matrix_synapse_worker_container_labels_public_metrics_middleware_basic_auth_enabled|default(true) %}
     basic_auth:
       username: prometheus
       password_file: /path/to/your/passwordfile.pwd