docs: def _fetch_rrd_meta(self, connection, rrd_path_root, whitelist, field_names, tags):
          ''' Fetch metadata about each RRD in this Cacti DB, returning a list of
              tuples of (hostname, device_name, rrd_path)
          '''
  
          def _in_whitelist(rrd):
              path = rrd.replace('<path_rra>/', '')
              for p in whitelist:
                  if fnmatch(path, p):
                      return True
              return False
  
          c = connection.cursor()
  
          and_parameters = " OR ".join(["hsc.field_name = '%s'" % field_name for field_name in field_names])
  
          # Check for the existence of the `host_snmp_cache` table
          rrd_query = """
              SELECT
                  h.hostname as hostname,
                  hsc.field_value as device_name,
                  dt.data_source_path as rrd_path
              FROM data_local dl
                  JOIN host h on dl.host_id = h.id
                  JOIN data_template_data dt on dt.local_data_id = dl.id
                  LEFT JOIN host_snmp_cache hsc on h.id = hsc.host_id
                      AND dl.snmp_index = hsc.snmp_index
              WHERE dt.data_source_path IS NOT NULL
              AND dt.data_source_path != ''
              AND ({} OR hsc.field_name is NULL) """.format(
              and_parameters
          )
  
          c.execute(rrd_query)
          res = []
          for hostname, device_name, rrd_path in c.fetchall():
              if not whitelist or _in_whitelist(rrd_path):
                  if hostname in ('localhost', '127.0.0.1'):
                      hostname = self.hostname
                  rrd_path = rrd_path.replace('<path_rra>', rrd_path_root)
                  device_name = device_name or None
                  res.append((hostname, device_name, rrd_path))
  
          # Collect stats
          num_hosts = len(set([r[0] for r in res]))
          self.gauge('cacti.rrd.count', len(res), tags=tags)
          self.gauge('cacti.hosts.count', num_hosts, tags=tags)
  
          return res