fix(xds): don't read metadata in ProxyBuilders (backport #5414)

[mergify]

This is an automatic backport of pull request #5414 done by Mergify.
Cherry-pick of 39ba902 has failed:

On branch mergify/bp/release-2.0/pr-5414
Your branch is up to date with 'origin/release-2.0'.

You are currently cherry-picking commit 39ba902fa.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   pkg/plugins/runtime/gateway/suite_test.go
	modified:   pkg/xds/server/v3/snapshot_generator_test.go
	modified:   pkg/xds/sync/dataplane_proxy_builder.go
	modified:   pkg/xds/sync/dataplane_watchdog.go
	modified:   pkg/xds/sync/dataplane_watchdog_test.go
	modified:   pkg/xds/sync/proxy_builder_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   pkg/api-server/inspect_endpoints.go
	both modified:   pkg/xds/sync/components.go
	both modified:   pkg/xds/sync/egress_proxy_builder.go
	both modified:   pkg/xds/sync/ingress_proxy_builder.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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