we index from 0 apparently

roles/matrix-prometheus/templates/prometheus.yml.j2

@@ -34,7 +34,7 @@ scrape_configs:
       labels:
         instance: {{ matrix_domain }}
         job: master
-        index: 1
+        index: 0
   {% for worker in matrix_synapse_workers_enabled_list|d([]) %}
   {% if worker.metrics_port != 0 %}
     - targets: ['matrix-synapse-worker-{{ worker.type }}-{{ worker.instanceId }}:{{ worker.metrics_port }}']