Prometheus: Migrate metadata queries to use resource calls (#49921)

* Sent resource calls for metadata to the backend

* moved resource calls to the backend

* code review feedback

* fixed post with body

* statuscode >= 300

* cleanup

* fixed tests

* fixed datasource tests

* code review feedback

* force some other endpoints to only GET

* fix linting errors

* fixed tests

* was able to remove section of redundant code

* cleanup and code review feedback

* moved query_exemplars to get request

* fixed return on error

* went back to resource calls, but using the backendsrv directly

* moved to a resource call with fallback

* fixed tests

* check for proper messages

* proper check for invalid calls

* code review changes

pkg/tsdb/prometheus/prometheus.go

@@ -16,6 +16,7 @@ import (
 	"github.com/grafana/grafana/pkg/setting"
 	"github.com/grafana/grafana/pkg/tsdb/prometheus/buffered"
 	"github.com/grafana/grafana/pkg/tsdb/prometheus/querydata"
+	"github.com/grafana/grafana/pkg/tsdb/prometheus/resource"
 	apiv1 "github.com/prometheus/client_golang/api/prometheus/v1"
 )
 
@@ -29,6 +30,7 @@ type Service struct {
 type instance struct {
 	buffered  *buffered.Buffered
 	queryData *querydata.QueryData
+	resource  *resource.Resource
 }
 
 func ProvideService(httpClientProvider httpclient.Provider, cfg *setting.Cfg, features featuremgmt.FeatureToggles, tracer tracing.Tracer) *Service {
@@ -57,9 +59,15 @@ func newInstanceSettings(httpClientProvider httpclient.Provider, cfg *setting.Cf
 			return nil, err
 		}
 
+		r, err := resource.New(httpClientProvider, cfg, features, settings, plog)
+		if err != nil {
+			return nil, err
+		}
+
 		return instance{
 			buffered:  b,
 			queryData: qd,
+			resource:  r,
 		}, nil
 	}
 }
@@ -81,6 +89,27 @@ func (s *Service) QueryData(ctx context.Context, req *backend.QueryDataRequest)
 	return i.buffered.ExecuteTimeSeriesQuery(ctx, req)
 }
 
+func (s *Service) CallResource(ctx context.Context, req *backend.CallResourceRequest, sender backend.CallResourceResponseSender) error {
+	i, err := s.getInstance(req.PluginContext)
+	if err != nil {
+		return err
+	}
+
+	statusCode, bytes, err := i.resource.Execute(ctx, req)
+	body := bytes
+	if err != nil {
+		body = []byte(err.Error())
+	}
+
+	return sender.Send(&backend.CallResourceResponse{
+		Status: statusCode,
+		Headers: map[string][]string{
+			"content-type": {"application/json"},
+		},
+		Body: body,
+	})
+}
+
 func (s *Service) getInstance(pluginCtx backend.PluginContext) (*instance, error) {
 	i, err := s.im.Get(pluginCtx)
 	if err != nil {