How do I add a _meta field to monitored files

I am working with a heavy forwarder tier that is running syslog where network devices are sending data. For ease of tracking where each file is being monitored from, I would like to add some metadata to the monitored files, that includes the heavy forwarder they are being collected from (this tier is load balanced so the data could land on any number of hosts). I have tried adding _meta = hvy_fwd::$HOSTNAME and that does not appear to be doing the trick. Example monitoring stanza: [monitor:///var/log/remote/my_network_device] index = network sourcetype = mysourcetype ignoreOlderThan = 1d disabled = false host_segment = 4 blacklist = \.(gz|tgz|xz|\d{1})$ _meta = heavy_forwarder::$HOSTNAME Any help would be greatly appreciated.