Serialize floating values locale-independent

Closes: #281

core/src/text_serializer.cc

@@ -2,7 +2,9 @@
 
 #include <cmath>
 #include <limits>
+#include <locale>
 #include <ostream>
+#include <sstream>
 
 namespace prometheus {
 
@@ -16,7 +18,12 @@ std::string ToString(double v) {
   if (std::isinf(v)) {
     return (v < 0 ? "-Inf" : "+Inf");
   }
-  return std::to_string(v);
+  std::ostringstream ss;
+  ss.imbue(std::locale::classic());
+  ss.setf(std::ios::fixed, std::ios::floatfield);
+  ss.precision(6);
+  ss << v;
+  return ss.str();
 }
 
 const std::string& EscapeLabelValue(const std::string& value,

core/tests/CMakeLists.txt

@@ -7,6 +7,7 @@ add_executable(prometheus_test
   gauge_test.cc
   histogram_test.cc
   registry_test.cc
+  serializer_test.cc
   summary_test.cc
   utils_test.cc
 )

core/tests/serializer_test.cc

@@ -0,0 +1,30 @@
+#include "prometheus/counter.h"
+#include "prometheus/family.h"
+#include "prometheus/text_serializer.h"
+
+#include <gmock/gmock.h>
+#include <locale>
+
+namespace prometheus {
+namespace {
+
+TEST(SerializerTest, shouldSerializeLocaleIndependent) {
+  Family<Counter> family{"requests_total", "", {}};
+  auto& counter = family.Add({});
+  counter.Increment();
+  const auto collected = family.Collect();
+
+  // save and change locale
+  const std::locale oldLocale = std::locale::classic();
+  std::locale::global(std::locale("de_DE.UTF-8"));
+
+  TextSerializer textSerializer;
+  const auto serialized = textSerializer.Serialize(collected);
+  EXPECT_THAT(serialized, testing::HasSubstr("1.0"));
+
+  // restore locale
+  std::locale::global(oldLocale);
+}
+
+}  // namespace
+}  // namespace prometheus