loki backend mode forward-oauth (#48401)

* loki: backend: add forward oauth credentials functionality

* removed obsolete comment

pkg/tsdb/loki/api_mock.go

@@ -0,0 +1,41 @@
+package loki
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+)
+
+type mockRequestCallback func(req *http.Request)
+
+type mockedRoundTripper struct {
+	statusCode      int
+	responseBytes   []byte
+	contentType     string
+	requestCallback mockRequestCallback
+}
+
+func (mockedRT *mockedRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	requestCallback := mockedRT.requestCallback
+	if requestCallback != nil {
+		requestCallback(req)
+	}
+
+	header := http.Header{}
+	header.Add("Content-Type", mockedRT.contentType)
+	return &http.Response{
+		StatusCode: mockedRT.statusCode,
+		Header:     header,
+		Body:       io.NopCloser(bytes.NewReader(mockedRT.responseBytes)),
+	}, nil
+}
+
+func makeMockedAPI(statusCode int, contentType string, responseBytes []byte, requestCallback mockRequestCallback) *LokiAPI {
+	client := http.Client{
+		Transport: &mockedRoundTripper{statusCode: statusCode, contentType: contentType, responseBytes: responseBytes, requestCallback: requestCallback},
+	}
+
+	return newLokiAPI(&client, "http://localhost:9999", log.New("test"), "")
+}