fix pd role metric for 3.0 (pingcap#1065)

Author: liubo0127

scripts/pd.json

@@ -137,7 +137,7 @@
           "tableColumn": "",
           "targets": [
             {
-              "expr": "count(delta(pd_tso_events{type=\"save\",instance=\"$instance\"}[1m]))",
+              "expr": "count(delta(pd_server_tso{type=\"save\",instance=\"$instance\"}[1m]))",
               "format": "time_series",
               "intervalFactor": 2,
               "legendFormat": "",
@@ -7243,4 +7243,4 @@
   "title": "Test-Cluster-PD",
   "uid": "Q6RuHYIWk",
   "version": 5
-}
+}