[demo] append to otel resource attributes

[puckpuck]

Allows users to completely override the OTEL_RESOURCE_ATTRIBUTES environment variable at the component level or append to it using a special OTEL_RESOURCE_ATTRIBUTES_EXTRA environment variable.

Users can now specify:

components:
  frontendProxy:
    envOverrides:
      - name: OTEL_RESOURCE_ATTRIBUTES_EXTRA
        value: "net.component=proxy"

And the resource attributes for the frontendProxy will also contain a net.component=proxy attrbribute.