Restore support for setting null as a tag value (Resolves #823) (#824)

Author: amirhadadi

jaeger-core/src/main/java/io/jaegertracing/internal/JaegerSpan.java

@@ -68,7 +68,7 @@ protected JaegerSpan(
     this.startTimeMicroseconds = startTimeMicroseconds;
     this.startTimeNanoTicks = startTimeNanoTicks;
     this.computeDurationViaNanoTicks = computeDurationViaNanoTicks;
-    this.tags = new ConcurrentHashMap<String, Object>();
+    this.tags = new ConcurrentHashMap<>();
     this.references = references != null ? new ArrayList<Reference>(references) : null;
 
     // Handle SAMPLING_PRIORITY tag first, as this influences whether setTagAsObject actually
@@ -111,7 +111,7 @@ public List<Reference> getReferences() {
   }
 
   public Map<String, Object> getTags() {
-    return new HashMap<String, Object>(tags);
+    return new HashMap<>(tags);
   }
 
   @Override
@@ -225,7 +225,7 @@ private JaegerSpan setTagAsObject(String key, Object value) {
     }
 
     if (context.isSampled()) {
-      tags.put(key, value);
+      tags.put(key, value == null ? "null" : value);
     }
 
     return this;

jaeger-core/src/test/java/io/jaegertracing/internal/JaegerSpanTest.java

@@ -124,6 +124,14 @@ public void testSetAndGetBaggageItem() {
     assertEquals(value, jaegerSpan.getBaggageItem(key));
   }
 
+  @Test
+  public void testSetNullTag() {
+    String key = "tag.key";
+
+    jaegerSpan.setTag(key, (String)null);
+    assertEquals("null", jaegerSpan.getTags().get(key));
+  }
+
   @Test
   public void testSetBooleanTag() {
     Boolean expected = true;