Documentation/op-guide: fix failed RPC rate, leader election metrics

This fixes failed RPC rate query, where we do not need
subtraction because we already query by the status code.
Also adds grpc_method to make it more specific. Most of the
time, the failure recovers within 10-second, which is our
Prometheus scrap interval, so 'rate' query might not cover
that time window, showing as 0s, but still shows up in the graph.

Current leader election metrics just displays the counter
vector, but if we want to 'Rate Leader Election', it should
query with 'rate' function.

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

Documentation/op-guide/grafana.json

@@ -123,9 +123,9 @@
                             "step": 2
                         },
                         {
-                            "expr": "sum(rate(grpc_server_started_total{grpc_type=\"unary\"} [1m])) - sum(rate(grpc_server_handled_total{grpc_type=\"unary\",grpc_code!=\"OK\"} [1m]))",
+                            "expr": "rate(grpc_server_handled_total{grpc_type=\"unary\",grpc_code!=\"OK\"}[1m])",
                             "intervalFactor": 2,
-                            "legendFormat": "{{instance}} RPC Failed Rate",
+                            "legendFormat": "{{instance}} {{grpc_method}} RPC Failed Rate",
                             "metric": "grpc_server_handled_total",
                             "refId": "B",
                             "step": 2
@@ -922,7 +922,7 @@
                     "stack": false,
                     "steppedLine": false,
                     "targets": [{
-                        "expr": "etcd_server_leader_changes_seen_total",
+                        "expr": "delta(etcd_server_leader_changes_seen_total[1m])",
                         "intervalFactor": 2,
                         "legendFormat": "{{instance}} Leader Change Seen",
                         "metric": "etcd_server_leader_changes_seen_total",
@@ -1009,4 +1009,4 @@
     "version": 215,
     "links": [],
     "gnetId": null
-}
+}