Evolve service.instance.id into a default, always present attribute

[jpkrohling]

As a follow-up from open-telemetry/opentelemetry-specification#3222, I'd like to propose a series of steps to make service.instance.id available by default for all SDKs. We may get into a situation where the final goal cannot be achieved, but I think we can go very far. Here's my proposal:

• Define a common algorithm to generate service.instance.id values, having it as experimental and optional at first, allowing SDKs to have this on by default
• Get it implemented by three SDKs, having this on by default on them
• If we determine that this doesn't cause adverse side-effects, evolve this to stable and mark as required

While having a good quality service.instance.id is nice to have for traces, it is crucial for metrics, as it's hard to tell metrics apart based on their origin without this kind of information. Right now, we can use things similar to target_info, but those are still not authoritative enough.

[joaopgrassi]

A use case where service.instance.id not being present "break" things: #519