truncateExtLabels support Unicode cut

[mickeyzzc]

in case : 《Thanos Query truncate ExtLabels does not support unicode #6266》

I came across a scenario where the upstream of the thanos query had a long label and mixed with Unicode.

When cutting, it just cuts to Unicode, causing the entire thanos query to crash.

This is a random patchwork, not necessarily 1000. But where it's cut, it happens to be a Unicode
，It look like：

{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云京", prometheus="monitoring/business", prometheus_replica="prometheus-business-0"},{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云北京", prometheus="monitoring/istio-telemetry", prometheus_replica="prometheus-istio-telemetry-0"},{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云北京", prometheus="monitoring/istio-telemetry", prometheus_replica="prometheus-istio-telemetry-shard-1-0"},{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云北京", prometheus="monitoring/istio-telemetry", prometheus_replica="prometheus-istio-telemetry-shard-2-0"},{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云北京", prometheus="monitoring/istio-telemetry", prometheus_replica="prometheus-istio-telemetry-shard-3-0"},{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云北京", prometheus="monitoring/istio-telemetry", prometheus_replica="prometheus-istio-telemetry-shard-4-0"},{cluster="k8s-hw-bj", idc_id="huaweicloud_bj", idc_name="华为云北`

The problem code is as follows：

// thanos/pkg/query/endpointset.go  :  v0.30 - v0.31
// truncateExtLabels truncates the stringify external labels with the format of {labels..}.
func truncateExtLabels(s string, threshold int) string {
	if len(s) > threshold {
		return fmt.Sprintf("%s}", s[:threshold-1])
	}
	return s
}

[saswatamcode]

Thanks! Mind adding a test case for this?

[mickeyzzc]

Thanks! Mind adding a test case for this?

I'll add later

[mickeyzzc]

This code fixes the problem：

goroutine 80141 [running]:
github.com/prometheus/client_golang/prometheus.MustNewConstMetric(...)
	/go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/value.go:130
github.com/thanos-io/thanos/pkg/query.(*endpointSetNodeCollector).Collect(0xc000af4b40, 0xc019970f60?)
	/app/pkg/query/endpointset.go:263 +0x2db
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()
	/go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/registry.go:456 +0x10d
created by github.com/prometheus/client_golang/prometheus.(*Registry).Gather
	/go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/registry.go:467 +0x55d

// /go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/value.go:130
// MustNewConstMetric is a version of NewConstMetric that panics where
// NewConstMetric would have returned an error.
func MustNewConstMetric(desc *Desc, valueType ValueType, value float64, labelValues ...string) Metric {
	m, err := NewConstMetric(desc, valueType, value, labelValues...)
	if err != nil {
		panic(err)
	}
	return m
}

func NewConstMetric(desc *Desc, valueType ValueType, value float64, labelValues ...string) (Metric, error) {
	if desc.err != nil {
		return nil, desc.err
	}
	if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
		return nil, err
	}

	metric := &dto.Metric{}
	if err := populateMetric(valueType, value, MakeLabelPairs(desc, labelValues), nil, metric); err != nil {
		return nil, err
	}

	return &constMetric{
		desc:   desc,
		metric: metric,
	}, nil
}

func validateLabelValues(vals []string, expectedNumberOfValues int) error {
	if len(vals) != expectedNumberOfValues {
		return fmt.Errorf(
			"%w: expected %d label values but got %d in %#v",
			errInconsistentCardinality, expectedNumberOfValues,
			len(vals), vals,
		)
	}

	for _, val := range vals {
		if !utf8.ValidString(val) {
			return fmt.Errorf("label value %q is not valid UTF-8", val)
		}
	}

	return nil
}

[saswatamcode]

LGTM!