Add method to filesinks to return filename

include/spdlog/sinks/basic_file_sink.h

@@ -30,6 +30,11 @@ public:
         file_helper_.open(filename, truncate);
     }
 
+    const filename_t &filename() const
+    {
+        return file_helper_.filename();
+    }
+
 protected:
     void sink_it_(const details::log_msg &msg) override
     {