Queries return same series twice with non-stringlabels build

[jDomantas]

What did you do?

In some cases range queries return multiple separate series with identical label sets where only one series is expected.

The conditions seem to be:

• metric needs to have at least one label lexicographically smaller than __name__, for example __address__
• metric needs to have at least one label lexicographically larger than __name__
• query needs to drop metric name, for example metric * 2
• it needs to be a range query

Here's a unit test showing the problem: jDomantas@171c965

Test error shows that prom engine returns two metrics - first one contains the first datapoint, and the second one contains the rest. Second metric has a duplicate label, which disappears when querying the api because response represents label set as a map.

Error:      	Not equal:
            	expected: {__address__="bar", job="1"} =>
            	200 @[0]
            	200 @[60000]
            	200 @[120000]
            	actual  : {__address__="bar", job="1"} =>
            	200 @[0]
            	{__address__="bar", job="1", job="1"} =>
            	200 @[60000]
            	200 @[120000]  promql.Matrix{
            	  	{
            	  		Metric: s`{__address__="bar", job="1"}`,
            	  		Floats: []promql.FPoint{
            	  			{F: 200},
            	- 			s"200 @[60000]",
            	- 			s"200 @[120000]",
            	  		},
            	  		Histograms: nil,
            	  	},
            	+ 	s"{__address__=\"bar\", job=\"1\", job=\"1\"} =>\n200 @[60000]\n200 @[120000]",
            	  }
Test:       	TestRangeQuery/duplicate-result

This bug appears to be introduced in #13446 - DropMetricName modifies Labels slice in-place, which sometimes screws up labels for other samples.

What did you expect to see?

No duplicate series returned

What did you see instead? Under which circumstances?

First datapoint of the range query result returned as separate series

System information

No response

Prometheus version

Prometheus version v2.51.0-rc.0 (used as a library)

Prometheus configuration file

No response

Alertmanager version

No response

Alertmanager configuration file

No response

Logs

No response

[beorn7]

@bboreham as you touched this in #13446, does this make sense to you?

With the upcoming arbitrary UTF-8 strings for label names, this might get triggered way more often…

[bboreham]

used as a library

Importantly, without -tags stringlabels, which release builds of Prometheus use.