DataSourceApi: convert interface to abstract class (#16979)

* DataSourceApi as class

* add diff signature

* Prometheus: moved directUrl to jsonData

pkg/api/frontendsettings.go

@@ -3,6 +3,7 @@ package api
 import (
 	"strconv"
 
+	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/util"
 
 	"github.com/grafana/grafana/pkg/bus"
@@ -86,12 +87,13 @@ func (hs *HTTPServer) getFrontendSettingsMap(c *m.ReqContext) (map[string]interf
 			defaultDatasource = ds.Name
 		}
 
-		if ds.JsonData != nil {
-			dsMap["jsonData"] = ds.JsonData
-		} else {
-			dsMap["jsonData"] = make(map[string]string)
+		jsonData := ds.JsonData
+		if jsonData == nil {
+			jsonData = simplejson.New()
 		}
 
+		dsMap["jsonData"] = jsonData
+
 		if ds.Access == m.DS_ACCESS_DIRECT {
 			if ds.BasicAuth {
 				dsMap["basicAuth"] = util.GetBasicAuthHeader(ds.BasicAuthUser, ds.DecryptedBasicAuthPassword())
@@ -123,7 +125,7 @@ func (hs *HTTPServer) getFrontendSettingsMap(c *m.ReqContext) (map[string]interf
 
 		if ds.Type == m.DS_PROMETHEUS {
 			// add unproxied server URL for link to Prometheus web UI
-			dsMap["directUrl"] = ds.Url
+			jsonData.Set("directUrl", ds.Url)
 		}
 
 		datasources[ds.Name] = dsMap