Make LogCategory::getPropagateLevelMessagesToParentRelaxed() const.

Summary:
There's no compelling reason I could uncover why someone with a const folly::LogCategory* shouldn't be able to get the propagation level, so I'm marking this method as const to allow for that.  This makes the method more consistent with all of the other get methods in the class.

For example, if one gets the category configuration map through getCategoryConfigs(), the map returned is const.

Reviewed By: yfeldblum

Differential Revision: D31066791

fbshipit-source-id: 46267ae7bb950fbd1b3c3d8b032ed2491d54e446

folly/logging/LogCategory.cpp

@@ -171,7 +171,7 @@ void LogCategory::setPropagateLevelMessagesToParent(LogLevel level) {
   propagateLevelMessagesToParent_.store(level, std::memory_order_relaxed);
 }
 
-LogLevel LogCategory::getPropagateLevelMessagesToParentRelaxed() {
+LogLevel LogCategory::getPropagateLevelMessagesToParentRelaxed() const {
   return propagateLevelMessagesToParent_.load(std::memory_order_relaxed);
 }
 

folly/logging/LogCategory.h

@@ -154,7 +154,7 @@ class LogCategory {
    * Get which messages processed by this category will be processed by the
    * parent category
    */
-  LogLevel getPropagateLevelMessagesToParentRelaxed();
+  LogLevel getPropagateLevelMessagesToParentRelaxed() const;
 
   /**
    * Get the LoggerDB that this LogCategory belongs to.