Make clear that the config has changed since prometheus v2

This restores the config that is usable for prometheus pre v2.0.0
The new config only works for Prometheus v2+

contrib/prometheus/README

@@ -5,7 +5,19 @@ To use it, first install prometheus by following the instructions at
 
   http://prometheus.io/
 
-Then add a new job to the main prometheus.yml file:
+### for Prometheus v1
+Add a new job to the main prometheus.conf file:
+
+  job: {
+    name: "synapse"
+
+    target_group: {
+      target: "http://SERVER.LOCATION.HERE:PORT/_synapse/metrics"
+    }
+  }
+
+### for Prometheus v2
+Add a new job to the main prometheus.yml file:
 
   - job_name: "synapse"
     metrics_path: "/_synapse/metrics"
@@ -18,7 +30,7 @@ Then add a new job to the main prometheus.yml file:
 To use `synapse.rules` add
 
     rule_files:
-      - "/PATH/TO/synapse.rules"
+      - "/PATH/TO/synapse-v2.rules"
 
 Metrics are disabled by default when running synapse; they must be enabled
 with the 'enable-metrics' option, either in the synapse config file or as a

contrib/prometheus/synapse-v1.rules

+synapse_federation_transaction_queue_pendingEdus:total = sum(synapse_federation_transaction_queue_pendingEdus or absent(synapse_federation_transaction_queue_pendingEdus)*0)
+synapse_federation_transaction_queue_pendingPdus:total = sum(synapse_federation_transaction_queue_pendingPdus or absent(synapse_federation_transaction_queue_pendingPdus)*0)
+
+synapse_http_server_requests:method{servlet=""} = sum(synapse_http_server_requests) by (method)
+synapse_http_server_requests:servlet{method=""} = sum(synapse_http_server_requests) by (servlet)
+
+synapse_http_server_requests:total{servlet=""} = sum(synapse_http_server_requests:by_method) by (servlet)
+
+synapse_cache:hit_ratio_5m = rate(synapse_util_caches_cache:hits[5m]) / rate(synapse_util_caches_cache:total[5m])
+synapse_cache:hit_ratio_30s = rate(synapse_util_caches_cache:hits[30s]) / rate(synapse_util_caches_cache:total[30s])
+
+synapse_federation_client_sent{type="EDU"} = synapse_federation_client_sent_edus + 0
+synapse_federation_client_sent{type="PDU"} = synapse_federation_client_sent_pdu_destinations:count + 0
+synapse_federation_client_sent{type="Query"} = sum(synapse_federation_client_sent_queries) by (job)
+
+synapse_federation_server_received{type="EDU"} = synapse_federation_server_received_edus + 0
+synapse_federation_server_received{type="PDU"} = synapse_federation_server_received_pdus + 0
+synapse_federation_server_received{type="Query"} = sum(synapse_federation_server_received_queries) by (job)
+
+synapse_federation_transaction_queue_pending{type="EDU"} = synapse_federation_transaction_queue_pending_edus + 0
+synapse_federation_transaction_queue_pending{type="PDU"} = synapse_federation_transaction_queue_pending_pdus + 0