`metric` field is useless in grafana json

[okJiang]

Enhancement Task

• metric field is useless and there is confusion with the expression.

for example

pd/metrics/grafana/pd.json

Lines 856 to 869 in 90273dd

 "targets": [ 
   { 
     "exemplar": true, 
     "expr": "service_member_role{k8s_cluster=\"$k8s_cluster\", tidb_cluster=\"$tidb_cluster\"}", 
     "format": "table", 
     "instant": true, 
     "interval": "", 
     "intervalFactor": 2, 
     "legendFormat": "", 
     "metric": "pd_server_tso", 
     "refId": "A", 
     "step": 40 
   } 
 ],