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
	}
	],