added level_t to sink. Improves the flexibility of loggers with multiple sinks

include/spdlog/sinks/sink.h

@@ -15,10 +15,33 @@ namespace sinks
 class sink
 {
 public:
+    sink(): _level( level::trace ) {}
+
     virtual ~sink() {}
     virtual void log(const details::log_msg& msg) = 0;
     virtual void flush() = 0;
+
+    bool should_log(level::level_enum msg_level) const;
+    void set_level(level::level_enum log_level);
+    level::level_enum level() const;
+
+private:
+    level_t _level;
+
 };
+
+inline bool sink::should_log(level::level_enum msg_level) const {
+    return msg_level >= _level.load(std::memory_order_relaxed);
+}
+
+inline void sink::set_level(level::level_enum log_level) {
+    _level.store(log_level);
+}
+
+inline level::level_enum sink::level() const {
+    return static_cast<spdlog::level::level_enum>(_level.load(std::memory_order_relaxed));
+}
+
 }
 }