[Star tree] Handle 'count', 'avg' metric and default metrics in star tree mapper

[bharath-techie]

Is your feature request related to a problem? Please describe

• Since all aggregation results has 'doc_count' as part response, star tree will require 'value_count' for all metric fields specified as part of star tree mapping
• When user specifies 'avg' metric, we need 'value_count' and 'sum' metric for the metric fields so that we can derive avg
• Set default metrics to be 'sum', 'value_count' and by extension 'avg' since it can be derived based on sum and count.
• Rename 'count' to 'value_count' in star tree mapping

Describe the solution you'd like

• Auto-add 'value_count' for all metric fields specified as part of star tree mapping
• To handle 'avg' metric, automatically add 'sum' metric to metric field so that we can compute during query time via 'sum/value_count'
• Set default metrics to be 'sum', 'value_count' and by extension 'avg' since it can be derived based on sum and count. Keeping it minimal since performance and storage size increases per every metric.
• Rename 'count' to 'value_count' in star tree flows to be inline with search aggregations

Related component

Indexing:Performance

Describe alternatives you've considered

No response

Additional context

No response