Remove long-deprecated instructions about promethesus console; also fix for modern config file format

docs/metrics-howto.rst

@@ -15,36 +15,11 @@ How to monitor Synapse metrics using Prometheus
 
   Restart synapse
 
-3: Check out synapse-prometheus-config
-  https://github.com/matrix-org/synapse-prometheus-config
+3: Add a prometheus target for synapse. It needs to set the ``metrics_path``
+   to a non-default value::
 
-4: Add ``synapse.html`` and ``synapse.rules``
-  The ``.html`` file needs to appear in prometheus's ``consoles`` directory,
-  and the ``.rules`` file needs to be invoked somewhere in the main config
-  file. A symlink to each from the git checkout into the prometheus directory
-  might be easiest to ensure ``git pull`` keeps it updated.
-
-5: Add a prometheus target for synapse
-  This is easiest if prometheus runs on the same machine as synapse, as it can
-  then just use localhost::
-
-    global: {
-      rule_file: "synapse.rules"
-    }
-
-    job: {
-      name: "synapse"
-
-      target_group: {
-        target: "http://localhost:9092/"
-      }
-    }
-
-6: Start prometheus::
-
-   ./prometheus -config.file=prometheus.conf
-
-7: Wait a few seconds for it to start and perform the first scrape,
-   then visit the console:
-
-    http://server-where-prometheus-runs:9090/consoles/synapse.html
+    - job_name: "synapse"
+      metrics_path: "/_synapse/metrics"
+      static_configs:
+        - targets:
+            "my.server.here:9092"