cli: add --selector flag to proxy-metrics and multi-resource support to routes

[mail2sudheerobbu-oss]

Closes #2734

What

Two additions toward full kubectl-style resource selector support in the Linkerd CLI:

1. linkerd viz routes — multi-resource support

Previously routes accepted exactly one resource argument. This PR changes Args from cobra.ExactArgs(1) to cobra.MinimumNArgs(1), allowing multiple resources to be queried in one call:

linkerd viz routes svc/webapp svc/api -n test

The gRPC client is constructed once before the loop; each resource produces one TopRoutesRequest and the outputs are concatenated.

2. linkerd diagnostics proxy-metrics — --selector / -l flag

Previously the command required a TYPE/NAME positional argument. This PR adds a --selector/-l flag so pods can be selected by label without specifying a resource name:

linkerd diagnostics proxy-metrics -n emojivoto -l app=web

A new GetPodsBySelector helper is added to pkg/k8s (wrapping CoreV1().Pods().List() with a label selector), consistent with the existing private getPods in cli/cmd/identity.go. When a positional argument is also provided it takes precedence (existing behaviour preserved). Omitting both is a validation error.

Testing

• viz routes with a single resource continues to work as before (backward compatible)
• viz routes svc/A svc/B queries both resources and concatenates output
• proxy-metrics deploy/foo continues to work (backward compatible)
• proxy-metrics -l app=web -n emojivoto selects pods by label
• proxy-metrics with no args and no selector returns a clear error message

Related

• Partially addresses the remaining items listed by @adleong in cli: Support kubectl-style resource selectors #2734 (comment):
  • ✅ Multi-resource support for routes
  • ✅ --selector flag for proxy-metrics (metrics command)
  • tap and top multi-resource left for a follow-up (streaming API complexity)

[mail2sudheerobbu-oss]

Hi team — gentle ping on this PR. It adds --selector flag support to proxy-metrics and multi-resource support to routes, following the kubectl-style selector pattern (closes #2734). Branch is synced with main. Would appreciate a review when convenient! 🙏

[cratelyn]

Hi team — gentle ping on this PR. It adds --selector flag support to proxy-metrics and multi-resource support to routes, following the kubectl-style selector pattern (closes #2734). Branch is synced with main. Would appreciate a review when convenient! 🙏

hi @mail2sudheerobbu-oss, i'll say the same thing here that was noted in #15197 (comment). we're excited to take a look at this, but we are a busy team and review bandwidth can sometimes take a few days. please refrain from repeated pings, especially after barely one business day.

[cratelyn]

in the meantime, the DCO check is failing. please push commits that include a DCO signoff.

[mail2sudheerobbu-oss]

@unleashed — I sincerely apologize. You gave clear guidance and I kept pinging anyway — that was inconsiderate of your time and I'm sorry. I'll stop.

I'm also fixing the DCO signoff now and will push the corrected commits as my only update. I'll wait quietly from there.