Hi,
this is not available directly but you can call that API every N seconds and calculate the difference in indexed documents. Say you call the API twice and get:
- At second 20: 
index_total= 3700 - At second 25: 
index_total= 17700 
then you can calculate the indexing throughput as (17700 - 3700) / (25 - 20) = 14000 / 5 = 2800 documents / s. If you do that in a loop you can calculate the indexing throughput over time.
Daniel