Update stopwatch.h (#3034)

Adding elapsed time in milliseconds.

include/spdlog/stopwatch.h

@@ -39,6 +39,10 @@ public:
         return std::chrono::duration<double>(clock::now() - start_tp_);
     }
 
+    std::chrono::milliseconds elapsed_ms() const {
+        return std::chrono::duration_cast<std::chrono::milliseconds>(clock::now() - start_tp_);
+    }
+
     void reset() { start_tp_ = clock::now(); }
 };
 }  // namespace spdlog