Use std::less<> for Labels to enable heterogeneous lookup on C++14+

Author: aeroyorch

core/include/prometheus/labels.h

@@ -1,12 +1,22 @@
 #pragma once
 
+#if defined(__cpp_lib_generic_associative_lookup)
+#include <functional>
+#endif
 #include <map>
 #include <string>
 
 namespace prometheus {
 
 /// \brief Multiple labels and their value.
+///
+/// Uses the transparent comparator std::less<> on C++14 and newer to allow
+/// heterogeneous lookup, falling back to the default comparator on C++11.
+#if defined(__cpp_lib_generic_associative_lookup)
+using Labels = std::map<std::string, std::string, std::less<>>;
+#else
 using Labels = std::map<std::string, std::string>;
+#endif
 
 /// \brief Single label and its value.
 using Label = Labels::value_type;