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/Manager.cc

@@ -1709,6 +1709,7 @@ WriterFrontend* Manager::CreateWriterForFilter(Filter* filter, const std::string
 
     auto* info = new WriterBackend::WriterInfo;
     info->path = util::copy_string(path.c_str(), path.size());
+    info->filter_name = filter->name;
     info->network_time = run_state::network_time;
 
     auto* filter_config_table = filter->config->AsTable();

src/logging/WriterBackend.h

@@ -67,6 +67,11 @@ public:
          */
         const char* path;
 
+        /**
+         * The filter this writer is attached to.
+         */
+        std::string filter_name;
+
         /**
          * The name of the postprocessor function that will be called
          * upon the logging manager processing the "rotation finished"
@@ -108,6 +113,8 @@ public:
 
             for ( config_map::const_iterator i = other.config.begin(); i != other.config.end(); i++ )
                 config.insert(std::make_pair(util::copy_string(i->first), util::copy_string(i->second)));
+
+            filter_name = other.filter_name;
         }
 
         ~WriterInfo() {

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.
      */