Some code refactoring in formatter

include/spdlog/details/fmt_helper.h

@@ -4,6 +4,9 @@
 
 #pragma once
 
+#include "chrono"
+#include "spdlog/fmt/fmt.h"
+
 // Some fmt helpers to efficiently format and pad ints and strings
 namespace spdlog {
 namespace details {
@@ -110,6 +113,18 @@ inline void pad6(size_t n, fmt::basic_memory_buffer<char, Buffer_Size> &dest)
     pad3(static_cast<int>(n % 1000), dest);
 }
 
+// return fraction of a second of the given time_point.
+// e.g.
+// fraction<std::milliseconds>(tp) -> will return the millis part of the second
+template<typename ToDuration>
+ToDuration time_fraction(const log_clock::time_point &tp)
+{
+    using namespace std::chrono;
+    auto duration = tp.time_since_epoch();
+    auto secs = duration_cast<seconds>(duration);
+    return duration_cast<ToDuration>(duration) - duration_cast<ToDuration>(secs);
+}
+
 } // namespace fmt_helper
 } // namespace details
 } // namespace spdlog