[Enhancement][Build-TimeReduction V1] Remove conversion of KNN Field to string if stored Fields not required

[navneet1v]

Description

While doing some benchmarks with Opensearch k-NN. I see that there is time going in to convert the kNN field to String even when storedFields is marked as false. On doing deep-dive I found out that, we convert the field to the string and then check if stored field is set as true or false.

Code ref:
https://github.com/opensearch-project/k-NN/blob/main/src/main/java/org/opensearch/knn/index/mapper/KNNVectorFieldMapper.java#L575

Here point.toString will convert the object to string which is expensive operation in high dimensionality use cases.

Refer the Flame graph for the same for Vector dimension: 1536 ingestion

[navneet1v]

The fix for the issue is pretty simple rather than doing toString we just need to pass the VectorField to the underline function.

[navneet1v]

Resolving the issue as the PR is merged.