Use cache for http.client in tsdb package. (#6833)

* datasource: move caching closer to datasource struct

* tsdb: use cached version of datasource http transport

closes #6825

pkg/api/metrics.go

@@ -8,6 +8,7 @@ import (
 	"github.com/grafana/grafana/pkg/api/dtos"
 	"github.com/grafana/grafana/pkg/metrics"
 	"github.com/grafana/grafana/pkg/middleware"
+	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/tsdb"
 	"github.com/grafana/grafana/pkg/tsdb/testdata"
 	"github.com/grafana/grafana/pkg/util"
@@ -25,9 +26,9 @@ func QueryMetrics(c *middleware.Context, reqDto dtos.MetricRequest) Response {
 			MaxDataPoints: query.Get("maxDataPoints").MustInt64(100),
 			IntervalMs:    query.Get("intervalMs").MustInt64(1000),
 			Model:         query,
-			DataSource: &tsdb.DataSourceInfo{
-				Name:     "Grafana TestDataDB",
-				PluginId: "grafana-testdata-datasource",
+			DataSource: &models.DataSource{
+				Name: "Grafana TestDataDB",
+				Type: "grafana-testdata-datasource",
 			},
 		})
 	}