docs: add snippet to forecast future time series in the Forecast a single time series with a univariate model tutorial (#1271)

* docs: add snippet to forecast future time series for a univariate model

* quick fix

* separate snippets

Author: rey-esp

samples/snippets/create_single_timeseries_forecasting_model_test.py

@@ -91,8 +91,22 @@ def test_create_single_timeseries() -> None:
     #  3	      0	              1	               1	      False	     -2470.632377	4945.264753	     44319.379307	        ['WEEKLY']	            False	        False	            True
     #  4	      2	              1	               1	      True	     -2463.671247	4937.342493	     42633.299513	        ['WEEKLY']	            False	        False	            True
     # [END bigquery_dataframes_single_timeseries_forecasting_model_tutorial_evaluate]
+
+    # [START bigquery_dataframes_single_timeseries_forecasting_model_tutorial_forecast]
+    prediction = model.predict(horizon=30, confidence_level=0.8)
+
+    print(prediction.peek())
+    # Expected output:
+    #           forecast_timestamp	   forecast_value	standard_error	confidence_level	prediction_interval_lower_bound	    prediction_interval_upper_bound	    confidence_interval_lower_bound	    confidence_interval_upper_bound
+    # 11	2017-08-13 00:00:00+00:00	1845.439732	      328.060405	      0.8	                 1424.772257	                      2266.107208	                     1424.772257	                     2266.107208
+    # 29	2017-08-31 00:00:00+00:00	2615.993932	      431.286628	      0.8	                 2062.960849	                      3169.027015	                     2062.960849	                     3169.027015
+    # 7	    2017-08-09 00:00:00+00:00	2639.285993	      300.301186	      0.8	                 2254.213792	                      3024.358193	                     2254.213792	                     3024.358193
+    # 25	2017-08-27 00:00:00+00:00	1853.735689	      410.596551	      0.8	                 1327.233216	                      2380.238162	                     1327.233216	                     2380.238162
+    # 1	    2017-08-03 00:00:00+00:00	2621.33159	      241.093355	      0.8	                 2312.180802	                      2930.482379	                     2312.180802	                     2930.482379
+    # [END bigquery_dataframes_single_timeseries_forecasting_model_tutorial_forecast]
     assert coef is not None
     assert summary is not None
     assert model is not None
     assert parsed_date is not None
+    assert prediction is not None
     assert total_visits is not None