Merge pull request #357 from gdusbabek/drop_emtpy_index_batches

drop empty index batches.
only index when there are modules registered.
Do not track index-write metrics in DiscoveryWriter.
Track index batch sizes and how long they take.

Track sizes, even when they are zero.
But only write track duration when there is stuff to index.
Add unit to repose of Http reponse handler
Merge pull request #359 from gdusbabek/ES_metrics

Measure ElasticSearch write peformance
Merge pull request #360 from chinmay-gupte/chinmay/return_units_with_http_query_response

Add unit to the respose of Http Rollup Handler