Skip null tag value for exporter (#843)

(cherry picked from commit 7adb4f2)

Author: songy23

exporters/stats/stackdriver/src/main/java/io/opencensus/exporter/stats/stackdriver/StackdriverExportUtils.java

@@ -177,6 +177,9 @@ static Metric createMetric(View view, List<? extends TagValue> tagValues) {
     for (int i = 0; i < tagValues.size(); i++) {
       TagKey key = columns.get(i);
       TagValue value = tagValues.get(i);
+      if (value == null) {
+        continue;
+      }
       stringTagMap.put(key.getName(), value.asString());
     }
     builder.putAllLabels(stringTagMap);

exporters/stats/stackdriver/src/test/java/io/opencensus/exporter/stats/stackdriver/StackdriverExportUtilsTest.java

@@ -70,6 +70,8 @@ public class StackdriverExportUtilsTest {
   @Rule public final ExpectedException thrown = ExpectedException.none();
 
   private static final TagKey KEY = TagKey.create("KEY");
+  private static final TagKey KEY_2 = TagKey.create("KEY2");
+  private static final TagKey KEY_3 = TagKey.create("KEY3");
   private static final TagValue VALUE_1 = TagValue.create("VALUE1");
   private static final TagValue VALUE_2 = TagValue.create("VALUE2");
   private static final String MEASURE_UNIT = "us";
@@ -153,6 +155,41 @@ public void createMetric() {
                 .build());
   }
 
+  @Test
+  public void createMetric_skipNullTagValue() {
+    View view =
+        View.create(
+            Name.create(VIEW_NAME),
+            VIEW_DESCRIPTION,
+            MEASURE_DOUBLE,
+            DISTRIBUTION,
+            Arrays.asList(KEY, KEY_2, KEY_3),
+            CUMULATIVE);
+    assertThat(StackdriverExportUtils.createMetric(view, Arrays.asList(VALUE_1, null, VALUE_2)))
+        .isEqualTo(
+            Metric.newBuilder()
+                .setType("custom.googleapis.com/opencensus/" + VIEW_NAME)
+                .putLabels("KEY", "VALUE1")
+                .putLabels("KEY3", "VALUE2")
+                .build());
+  }
+
+  @Test
+  public void createMetric_throwWhenTagKeysAndValuesHaveDifferentSize() {
+    View view =
+        View.create(
+            Name.create(VIEW_NAME),
+            VIEW_DESCRIPTION,
+            MEASURE_DOUBLE,
+            DISTRIBUTION,
+            Arrays.asList(KEY, KEY_2, KEY_3),
+            CUMULATIVE);
+    List<TagValue> tagValues = Arrays.asList(VALUE_1, null);
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("TagKeys and TagValues don't have same size.");
+    StackdriverExportUtils.createMetric(view, tagValues);
+  }
+
   @Test
   public void convertTimestamp() {
     Timestamp censusTimestamp1 = Timestamp.create(100, 3000);

impl_core/src/main/java/io/opencensus/implcore/stats/MutableViewData.java

@@ -139,7 +139,7 @@ static List<TagValue> getTagValues(
     for (int i = 0; i < columns.size(); ++i) {
       TagKey tagKey = columns.get(i);
       if (!tags.containsKey(tagKey)) {
-        // replace not found key values by “unknown/not set”.
+        // replace not found key values by null.
         tagValues.add(UNKNOWN_TAG_VALUE);
       } else {
         tagValues.add(tags.get(tagKey));