Make TestLogHandler::getMessageValues const

Summary: I want to use TestLogHandler::getMessageValues with a const TestLogHandler reference. Mark getMessageValues as const so it can be called on a const TestLogHandler. getMessageValues doesn't mutate class state, so this should be safe.

Reviewed By: chadaustin

Differential Revision: D9815552

fbshipit-source-id: 275625d1a001fe536586229228eb2c9eee7e6330

folly/logging/test/TestLogHandler.cpp

@@ -19,7 +19,7 @@
 
 namespace folly {
 
-std::vector<std::string> TestLogHandler::getMessageValues() {
+std::vector<std::string> TestLogHandler::getMessageValues() const {
   std::vector<std::string> values;
   values.reserve(messages_.size());
   for (const auto& msgInfo : messages_) {

folly/logging/test/TestLogHandler.h

@@ -45,7 +45,7 @@ class TestLogHandler : public LogHandler {
     return messages_;
   }
 
-  std::vector<std::string> getMessageValues();
+  std::vector<std::string> getMessageValues() const;
   void clearMessages() {
     messages_.clear();
   }