[thanos] add "component: server" to sidecar selector

[belm0]

Q	A
Bug fix?	yes
New feature?	no
API breaks?	no
Deprecations?	no
Related tickets
License	Apache 2.0

What's in this PR?

Qualify the selector for the sidecar pod to the prometheus server component.

Why?

Sidecar is usually running with the server. Without this qualification, pods of other components like pushgateway or node_exporter will match.

[ahma]

Thanks @belm0 LGTM

[tarokkk]

We use the Thanos chart in harmony with Prometheus Operator Chart which installs prometheus with label set:

  labels:
    app: prometheus
    controller-revision-hash: prometheus-one-eye-prometheus-operato-prometheus-67b8b9db8
    prometheus: one-eye-prometheus-operato-prometheus
    statefulset.kubernetes.io/pod-name: prometheus-one-eye-prometheus-operato-prometheus-0

and node export with

  labels:
    app: prometheus-node-exporter
    chart: prometheus-node-exporter-1.8.1
    controller-revision-hash: 76f9b5cc47
    heritage: Helm
    jobLabel: node-exporter
    pod-template-generation: "1"
    release: one-eye-prometheus-operator

Considering this fixing a default value in the values file does not seem proper. Can you please describe your use case?

[belm0]

the official prometheus Helm Chart sets component to server (based on server.name), and app to prometheus.

https://github.com/helm/charts/blob/98d951ea63065349fa8a31f552376207fb6115e6/stable/prometheus/values.yaml#L541

If we don't select correctly on component, we'll get the wrong job.

Currently we have this explicit selector:

  selector:
    app: prometheus
    component: server

if there isn't an appropriate default that doesn't cause surprises, I understand this PR may not be viable.

But I'll argue that not selecting on component will sometimes work, sometimes not (depending on luck and the ordering of jobs). So having some default may be better, even if the user needs to alter it.

[tarokkk]

I agree that is better to have a concrete label selector and I'll suggest in a Prometheus Operator issue to use the official Kubernetes suggested labels for the components.