feat(charts): support extra relabelings on control-plane PodMonitors

[Coder-in-a-shell]

Problem

The linkerd-control-plane chart's templates/podmonitor.yaml ships a fixed set of relabelings on each of its three PodMonitors, and no metricRelabelings at all. Operators who need to add their own rules — the motivating case in #15212 is dropping a pod that a second, non-Prometheus scraper (vmagent) already covers, to avoid duplicate timeseries — have no way to do it from values and must fork the chart.

Solution

Adds extraRelabelings and extraMetricRelabelings under podMonitor.controller, podMonitor.serviceMirror and podMonitor.proxy.

This follows the shape @alpeb confirmed in #15212 (comment), and specifically the append semantics rather than a wholesale spec-override map: the extra rules land after the relabelings the chart already sets, so the labelmap/labeldrop chain that produces the expected proxy metric labels can't be replaced by accident.

Both values default to [], and the metricRelabelings block renders only when non-empty, so default output is byte-for-byte unchanged — the only golden-file churn is the two new empty keys in the linkerd-config values dump.

This is the same pattern as the two already-merged siblings on this template: podMonitor.labels (#11222) and honorTimestamps (#15080).

Validation

helm template with the exact drop rule from the issue, plus metric relabelings on all three PodMonitors, renders as intended:

podMonitor:
  enabled: true
  proxy:
    extraRelabelings:
      - sourceLabels:
          - __meta_kubernetes_namespace
          - __meta_kubernetes_pod_label_app_kubernetes_io_name
        action: drop
        regex: ^metrics;vmagent$
    extraMetricRelabelings:
      - sourceLabels: [__name__]
        action: drop
        regex: go_.*

Parsing the rendered manifests confirms placement and count for each PodMonitor:

linkerd-controller               | relabelings=4  last=pod_template_hash   | metricRelabelings=0
linkerd-multicluster-controller  | relabelings=3  last=                    | metricRelabelings=1
linkerd-proxy                    | relabelings=13 last=^metrics;vmagent$   | metricRelabelings=1

i.e. the extra rules are appended last and metricRelabelings appears only where configured. Rendering with the defaults produces output identical to main.

Also ran:

• go test ./pkg/charts/linkerd2/... — pass
• go test ./cli/cmd/... — pass (golden files regenerated with -update; diff is only the two new empty keys)
• go build ./..., go vet, gofmt — clean

Fixes #15212

Signed-off-by: Sudhir Jaiswal jaiswalsudhir2944@gmail.com