Fix include-what-you-use in exporter_utils_test.cc

Author: dashpole

exporters/prometheus/test/exporter_utils_test.cc

@@ -15,7 +15,7 @@
 #include "opentelemetry/common/timestamp.h"
 #include "opentelemetry/exporters/prometheus/exporter_utils.h"
 #include "opentelemetry/nostd/unique_ptr.h"
-#include "opentelemetry/sdk/common/attribute_utils.h"
+#include "opentelemetry/nostd/variant.h"
 #include "opentelemetry/sdk/instrumentationscope/instrumentation_scope.h"
 #include "opentelemetry/sdk/metrics/data/metric_data.h"
 #include "opentelemetry/sdk/metrics/data/point_data.h"