Datasources: Support mixed datasources in a single query (#56832)

* initial cut at refactor - need to run more tests

* fix unit tests

* change newly unused function to test helper

* create unit tests for parsing query requests that cover a range of cases

* add some comments

* rename function to avoid dev confusion

pkg/api/metrics.go

@@ -54,7 +54,7 @@ func (hs *HTTPServer) QueryMetricsV2(c *models.ReqContext) response.Response {
 
 	reqDTO.HTTPRequest = c.Req
 
-	resp, err := hs.queryDataService.QueryData(c.Req.Context(), c.SignedInUser, c.SkipCache, reqDTO, true)
+	resp, err := hs.queryDataService.QueryData(c.Req.Context(), c.SignedInUser, c.SkipCache, reqDTO)
 	if err != nil {
 		return hs.handleQueryMetricsError(err)
 	}