This PR enables stats on inference to be gathered and stored in the `.ml-stats-*` indices.

Each node + model_id will have its own running stats document and these will later be summed together when returning _stats to the user.

`.ml-stats-*` is ILM managed (when possible). So, at any point the underlying index could change. This means that a stats document that is read in and then later updated will actually be a new doc in a new index. This complicates matters as this means that having a running knowledge of seq_no and primary_term is complicated and almost impossible. This is because we don't know the latest index name.

We should also strive for throughput, as this code sits in the middle of an ingest pipeline (or even a query).