Document push api RawSample.ID field

1. It seems it's currently undocumented if the ID should be UUID. Later down the ingestion pipeline we seem to expect it to be UUID. We should document if the ID field should be UUID and what happens if it is empty or not UUID.

2. It looks like we currently ignore (do not pas further then infesters) the user provided ID field and only maybe include in the distributor errors.
We generate new UUID when pass a profile down the ingestion pipeline, both for ingesters and segment writers. I think it makes sense to use the user provided ID for at least one case when we send profiles to segment writers and there is no pprof split.