Thanos Query truncate ExtLabels does not support unicode

[mickeyzzc]

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
}

[mickeyzzc]

truncateExtLabels support Unicode cut #6267

[mickeyzzc]

Merged Branch