fix: use part-of kubernetes label instead of instance in demo chart

[Machyne]

The Kubernetes Recommended Labels docs say that instance should be a unique identifier for an instance of an application. Here, it's set to the helm installation name everywhere. I think part-of is more appropriate in this case.

Using instance creates issues when scraping service labels per the semantic conventions: since no resource.opentelemetry.io/service.name annotation is provided, this well known k8s label app.kubernetes.io/instance is checked next. The result is that all of the data from the workload pods sent through k8sattributes processor using the service.name metadata will come out with the same service.name (ex: "my-otel-demo" when using the example install command).

If you think this is an issue with the semantic conventions, please let me know and I will take this discussion to that repo instead. Thanks in advance!

(note: this is a one-line diff on charts/opentelemetry-demo/templates/_helpers.tpl plus the result of make generate-examples)