Expose the flusher thread object to user in order to allow setting of thread name and thread affinity when needed (#3009)

* Expose the flusher thread object to user in order to allow setting of thread name and thread affinity when needed

* Code review fix - periodic_worker thread getter should return a reference and not a pointer

include/spdlog/details/periodic_worker.h

@@ -40,6 +40,7 @@ public:
             }
         });
     }
+    std::thread &get_thread() { return worker_thread_; }
     periodic_worker(const periodic_worker &) = delete;
     periodic_worker &operator=(const periodic_worker &) = delete;
     // stop the worker thread and join it

include/spdlog/details/registry.h

@@ -66,6 +66,8 @@ public:
         periodic_flusher_ = std::make_unique<periodic_worker>(clbk, interval);
     }
 
+    std::unique_ptr<periodic_worker> &get_flusher() { std::lock_guard<std::mutex> lock(flusher_mutex_); return periodic_flusher_; }
+
     void set_error_handler(err_handler handler);
 
     void apply_all(const std::function<void(const std::shared_ptr<logger>)> &fun);