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cherry-pick #11680

In our multi cluster mesh, we found that, even we meet the namespace-sameness, cross cluster calls to remote cluster through eastwest gateway never happen in some cases. (calls always invoke to primary cluster)
After some digging, we found that the same kubernetes Service had different port.name, and the code that aggregating eds reads the port.name to match endpoint with service.
And if we correct the Service.spec.ports[_].name to be same as what in primary cluster (literally equal, both name no specified consider equal "" == "" ), cross cluster calls act as expect.
So as namespace-sameness, the addition constraint should be documented.

https://github.com/istio/istio/blob/93e4ec424b52da807228bd7784c59d646ec73733/pilot/pkg/xds/endpoint_builder.go#L240-L242

for _, ep := range endpoints {
			// TODO(nmittler): Consider merging discoverability policy with cluster-local
			if !ep.IsDiscoverableFromProxy(b.proxy) {
				continue
			}
			if svcPort.Name != ep.ServicePortName {
				continue
			}

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