[Data Explorer][Discover 2.0] Break down data fetch logic to multiple data streams or use Expression for histogram

[ananzh]

Currently in discover 2.0, we are re-fetch data when histogram interval is updated. Histogram interval only updates aggregation which won't affect total data fetch. It is a bit of waste to send the fetch query to OS. Solution could be breaking down the current data stream to separate streams for histogram and table or use Expression for histogram.