Minor api documentation cleanup (#728)

Author: pyohannes

api/docs/Doxyfile

@@ -839,8 +839,10 @@ RECURSIVE              = YES
 
 EXCLUDE                = ../include/opentelemetry/common/spin_lock_mutex.h \
                          ../include/opentelemetry/common/key_value_iterable_view.h \
+                         ../include/opentelemetry/common/kv_properties.h \
+                         ../include/opentelemetry/common/string_util.h \
                          ../include/opentelemetry/trace/span_context_kv_iterable_view.h \
-                         ../include/opentelemetry/nostd/detail
+                         ../include/opentelemetry/nostd
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded

api/include/opentelemetry/common/key_value_iterable.h

@@ -29,23 +29,5 @@ class KeyValueIterable
    */
   virtual size_t size() const noexcept = 0;
 };
-
-//
-// NULL object pattern empty iterable.
-//
-class NullKeyValueIterable : public KeyValueIterable
-{
-public:
-  NullKeyValueIterable(){};
-
-  virtual bool ForEachKeyValue(
-      nostd::function_ref<bool(nostd::string_view, common::AttributeValue)>) const noexcept
-  {
-    return true;
-  };
-
-  virtual size_t size() const noexcept { return 0; }
-};
-
 }  // namespace common
 OPENTELEMETRY_END_NAMESPACE