[ML] Transform exponential_avg* stats not updated in batch mode

[davidkyle]

In this elided stats output from a batch transform the exponential_avg_* stats are all 0.0 which is confusing some indexing and processing has taken place.

"stats" : {
        "pages_processed" : 4,
        "documents_processed" : 50000,

        "exponential_avg_checkpoint_duration_ms" : 0.0,
        "exponential_avg_documents_indexed" : 0.0,
        "exponential_avg_documents_processed" : 0.0
      },

Either update the stats for batch mode or if not possible then do not display them.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)