[Star tree] Star tree indexing performance optimizations

[bharath-techie]

Is your feature request related to a problem? Please describe

Currently as part of star tree building during indexing, as part of off-heap build, we duplicate the metric field values for each metric stat, so if a metric field has 4 stats (sum, min, max, value_count) - we write 4 values into index output , instead of one.

Secondly we write individual dimension and metrics directly to IndexOutput.

Describe the solution you'd like

We can write only the actual field value from segments for each of the metric fields during flush.

We can buffer each starTreeDocument in memory and write bytes to indexOutput.

Star tree config

Dimensions

* Timestamp
    * Minute
    * Half Hour
* Target status code
* ELB status code

Metrics

* 8 field [ 4 stats each ]

Hers is how time taken to sort and aggregate segment documents to star-tree documents improved after these changes :

• Baseline - 24 seconds for 1.5 million documents
• Metric optimization - 10 seconds for 1.5 million documents
• Star tree document buffer optimization + Metric optimization - 2.5 seconds for 1.5 million documents

Related component

Search:Performance

Describe alternatives you've considered

No response

Additional context

No response