TCP ingest listener free slots for lines to be processed (high when idle, zero when over-loaded)

Example Metric Record:

json
{
   "#repo" : "humio-metrics",
   "#type" : "metrics",
   "@host" : "15",
   "@id" : "9iorL6KUunDPtXomKtgLGG7E_6237_1256_1726717345",
   "@ingesttimestamp" : "1726717345718",
   "@timestamp" : 1726717345689,
   "@timestamp.nanos" : "0",
   "@timezone" : "UTC",
   "name" : "ingest-listener-tcp-available",
   "type" : "GAUGE",
   "value" : "80000"
}