Query with a last empty equality label are not returning consistent response

[cyriltovena]

Query like this :

{app="prometheus",level=""}

Returns most of the time: {}

and sometimes it returns result but I would expect no label lvl.

{
"streams": [{
	"labels": "logs{app=\"prometheus\",  stream=\"stderr\"}",
	"entries": [{
		"ts": "2019-05-16T02:49:36.970126886Z",
		"line": "2019/05/16 02:49:36 Watching directory: \"/etc/config\"\n"
	}, {
		"ts": "2019-05-16T02:49:35.409789653Z",
		"line": "I0516 02:49:35.409635       1 main.go:211] Starting metrics server: 0.0.0.0:8080\n"
		}, {
		"ts": "2019-05-16T02:49:46.63968882Z",
		"line": "level=info ts=2019-05-16T02:49:46.639284922Z caller=cluster.go:587 component=cluster msg=\"gossip settled; proceeding\" elapsed=10.002738551s\n"
		},

If I filter with: {app="prometheus",level="info"}
I can verify that the entry for 2019-05-16T02:49:46.63968882Z has the label level and should have not been returned previously

{
"labels": "logs{app=\"prometheus\",  level=\"info\", stream=\"stderr\"}",
"entries": [ {
	"ts": "2019-05-16T02:49:46.63968882Z",
	"line": "level=info ts=2019-05-16T02:49:46.639284922Z caller=cluster.go:587 component=cluster msg=\"gossip settled; proceeding\" elapsed=10.002738551s\n"
},