Merge pull request #580 from adriancole/clarify-in-scope

Clarifies the contract around scoping a trace context

Author: sebright2

core/src/main/java/io/opencensus/tags/TagContexts.java

@@ -24,6 +24,10 @@
  *
  * <p>This class returns {@link TagContextBuilder builders} that can be used to create the
  * implementation-dependent {@link TagContext}s.
+ *
+ * <p>Implementations may have different constraints and are free to convert tag contexts to their
+ * own subtypes. This means callers cannot assume the {@link #getCurrentTagContext() current
+ * context} is the same instance as the one {@link #withTagContext(TagContext) placed into scope}.
  */
 public abstract class TagContexts {
   private static final TagContexts NOOP_TAG_CONTEXTS = new NoopTagContexts();