[demo] support custom default pod label in pod

[Frapschen]

support costum default pod label for istio injection case

[puckpuck]

Istio support was added about a month ago. Can you do a helm repo update and try again?

[Frapschen]

Istio support was https://github.com/open-telemetry/opentelemetry-helm-charts/pull/715 about a month ago.

https://github.com/open-telemetry/opentelemetry-helm-charts/pull/715 is about making service name align istio's standard, not about adding an istio sidecar inject label, if I'm wrong, please correct me.