Fix cluster variable in mixins

[QuentinBisson]

Fix mixin cluster parameter otherwise generated mixins looks like this:

{"type":"prometheus","uid":"${datasource}"},"label":"cluster","name":"job","query":"label_values(etcd_server_has_leader{job=~\".*etcd.*\"}, job)","refresh":2,"type":"query"}]},"time":{"from":"now-15m","to":"now"},"timezone": "`}}{{ .Values.grafana.defaultDashboardsTimezone }}{{`","title":"etcd","uid":"c2f4e12cdf69feb95caa41a5a1b423d9"}`}}

where name is job when the variable name used in dashboard queries is cluster.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

Issue came up here: prometheus-community/helm-charts#3880 (comment)

[QuentinBisson]

cc @ahrtr as I see you refactored to grafonnet recently

[jmhbnz]

Thanks for raising @QuentinBisson.

For context here is the original pr that introduced configurable cluster label: https://github.com/etcd-io/etcd/pull/13255/files

Potential regression when we refactored the mixin recently.

Can take a look at this tonight and provide some feedback.

cc @v-zhuravlev

[QuentinBisson]

Thank you @jmhbnz.

As you can see in this line https://github.com/etcd-io/etcd/pull/13255/files#diff-007651fd1693f51e6c72b237590b65d2a1a72fa3bba2d24993c2bf417bb25792R1294 the variable name used to be cluster but it is now set to config.clusterLabel which is job by default as you can see in my PR :)

[jmhbnz]

LGTM - Thanks for fixing this @QuentinBisson.

[ahrtr]

cc @v-zhuravlev to double confirm.

[v-zhuravlev]

@ahrtr , @QuentinBisson ,
my bad, tested this with cluster value of clusterLabel, therefore not realizing issue with other labels (such as job which is default).
Fix in this PR looks correct to me.

[ahrtr]

thx @v-zhuravlev for the quick confirmation.