From the spec

[service.instance.id] MUST be unique for each instance of the same service.namespace,service.name pair (in other words service.namespace,service.name,service.instance.id triplet MUST be globally unique). The ID helps to distinguish instances of the same service that exist at the same time (e.g. instances of a horizontally scaled service) [...] If the service has no inherent unique ID that can be used as the value of this attribute it is recommended to generate a random [UUID]

At the moment, just using the default resource in our SDK violates the requirement that "service.namespace,service.name,service.instance.id triplet MUST be globally unique":

In [1]: from opentelemetry.sdk.resources import Resource

In [2]: Resource.create().attributes
Out[2]: BoundedAttributes({'telemetry.sdk.language': 'python', 'telemetry.sdk.name': 'opentelemetry', 'telemetry.sdk.version': '1.5.0', 'service.name': 'unknown_service'}, maxlen=None)

This is very important for metrics where resource is part of the identity of a time series and we must not violate the single-writer principle.

1. Should we generate the service.instance.id from a UUID if it is not present? I think yes, so that we do not violate the global uniqueness.
2. Should we try to automagically populate this with a more meaningful value when it is present in a resource? E.g. container.id when present (this might be a spec question).
3. Since this is such a common use case, should we provide an easy way for the user to pass some of these key service.* attributes into the SDK if they don't want it generated? E.g. an environment variable, constructor kwarg to the Tracer/Meter provider, kwarg to the resource.