add a `FOLLY_INIT_LOGGING_CONFIG()` helper macro

Summary:
Add a macro to help users define the `getBaseLoggingConfig()` function.

While I would prefer to avoid macros if possible, this seems worthwhile.  This
saves 4 or 5 lines of boilerplate code in each program that sets a custom base
logger setting.  It also reduces the likelihood of a developer accidentally
having a typo in the function name, which would still build successfully but
not have the desired results.

Reviewed By: chadaustin

Differential Revision: D7457652

fbshipit-source-id: 1c316c7ea6949c16bd7b61c0440cc1ee69ecb83e

folly/experimental/logging/Init.h

@@ -77,3 +77,21 @@ void initLoggingOrDie(folly::StringPiece configString = "");
 const char* getBaseLoggingConfig();
 
 } // namespace folly
+
+/**
+ * A helper macro to set the default logging configuration in a program.
+ *
+ * This defines the folly::getBaseLoggingConfig() function, and makes it return
+ * the specified string.
+ *
+ * This macro should be used at the top-level namespace in a .cpp file in your
+ * program.
+ */
+#define FOLLY_INIT_LOGGING_CONFIG(config)            \
+  namespace folly {                                  \
+  const char* getBaseLoggingConfig() {               \
+    static constexpr StringPiece configSP((config)); \
+    return configSP.data();                          \
+  }                                                  \
+  }                                                  \
+  static_assert(true, "require a semicolon after FOLLY_INIT_LOGGING_CONFIG()")

folly/experimental/logging/example/main.cpp

@@ -29,15 +29,11 @@ using folly::LogLevel;
 // This will use default log settings defined by folly::initializeLoggerDB().
 static ExampleObject staticInitialized("static");
 
-namespace folly {
-const char* getBaseLoggingConfig() {
-  // Configure folly to enable INFO+ messages, and everything else to
-  // enable WARNING+.
-  //
-  // Set the default log handler to log asynchronously by default.
-  return ".=WARNING,folly=INFO; default:async=true";
-}
-} // namespace folly
+// Configure folly to enable INFO+ messages, and everything else to
+// enable WARNING+.
+//
+// Set the default log handler to log asynchronously by default.
+FOLLY_INIT_LOGGING_CONFIG(".=WARNING,folly=INFO; default:async=true");
 
 int main(int argc, char* argv[]) {
   // Using log macros before calling folly::initLogging() will use the default