logging: Add filter_name to WriterInfo

...with this change, it'll be possible to identify WriterFrontend's
based on (stream, filter_name, path) pairs in addition to (stream,
writer, path) pairs.

src/logging/WriterFrontend.h

@@ -238,6 +238,11 @@ public:
      */
     const char* Name() const { return name; }
 
+    /**
+     * Returns the name of the filter for which this frontend was instantiated.
+     */
+    const std::string& GetFilterName() const { return info->filter_name; }
+
     /**
      * Returns the log fields as passed into the constructor.
      */