Add Index Stats API (opensearch-project#4892)

* Add Index Stats API

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Reword for clarity

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Apply suggestions from code review

Co-authored-by: Nathan Bower <nbower@amazon.com>
Signed-off-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>

* Update _api-reference/index-apis/stats.md

Signed-off-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>

* Implemented editorial comments

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Implement editorial comments

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

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Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>
Signed-off-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>
Co-authored-by: Nathan Bower <nbower@amazon.com>