Plugins: Only set non-existing headers for core plugin requests (#78633)

* Only set non-existing headers

- Add test scenario

* Fix test

pkg/services/pluginsintegration/clientmiddleware/httpclient_middleware_test.go

@@ -276,6 +276,36 @@ func TestHTTPClientMiddleware(t *testing.T) {
 				require.Equal(t, "cookie2", reqClone.Cookies()[1].Name)
 				require.Equal(t, "cookie3", reqClone.Cookies()[2].Name)
 			})
+
+			t.Run("Should not overwrite an existing header", func(t *testing.T) {
+				_, err = cdt.Decorator.CheckHealth(req.Context(), &backend.CheckHealthRequest{
+					PluginContext: pluginCtx,
+					Headers:       headers,
+				})
+				require.NoError(t, err)
+				require.NotNil(t, cdt.CheckHealthReq)
+				require.Len(t, cdt.CheckHealthReq.Headers, 6)
+
+				middlewares := httpclient.ContextualMiddlewareFromContext(cdt.QueryDataCtx)
+				require.Len(t, middlewares, 1)
+				require.Equal(t, forwardPluginRequestHTTPHeaders, middlewares[0].(httpclient.MiddlewareName).MiddlewareName())
+
+				reqClone := req.Clone(req.Context())
+				// Create a header on the request as if it had been set by some other logic e.g. preceding middleware
+				reqClone.Header.Set(backend.OAuthIdentityTokenHeaderName, "bearer test-token")
+				res, err := middlewares[0].CreateMiddleware(httpclient.Options{}, finalRoundTripper).RoundTrip(reqClone)
+				require.NoError(t, err)
+				require.NoError(t, res.Body.Close())
+				require.Len(t, reqClone.Header, 5)
+				require.Equal(t, "true", reqClone.Header.Get(ngalertmodels.FromAlertHeaderName))
+				require.Equal(t, "bearer test-token", reqClone.Header.Get(backend.OAuthIdentityTokenHeaderName))
+				require.Equal(t, "id-token", reqClone.Header.Get(backend.OAuthIdentityIDTokenHeaderName))
+				require.Equal(t, "uname", reqClone.Header.Get(proxyutil.UserHeaderName))
+				require.Len(t, reqClone.Cookies(), 3)
+				require.Equal(t, "cookie1", reqClone.Cookies()[0].Name)
+				require.Equal(t, "cookie2", reqClone.Cookies()[1].Name)
+				require.Equal(t, "cookie3", reqClone.Cookies()[2].Name)
+			})
 		})
 	})
 }