test(pull): fix build with shared libs

Fix: #687

Author: gjasny

push/src/detail/label_encoder.h

@@ -3,11 +3,12 @@
 #include <iosfwd>
 
 #include "prometheus/labels.h"
+#include "prometheus/detail/push_export.h"
 
 namespace prometheus {
 namespace detail {
 
-void encodeLabel(std::ostream& os, const Label& label);
+PROMETHEUS_CPP_PUSH_EXPORT void encodeLabel(std::ostream& os, const Label& label);
 
 }
 }  // namespace prometheus