[Feature Request]: MongoDB doc_store add Index

[danerlt]

Feature Description

I am using MongoDB as the doc_store, and the structure stored within it is as follows:

{
  "_id": "2c5ae478-327c-4620-894e-f1a9cefde5b8",
  "__data__": {
    "id_": "2c5ae478-327c-4620-894e-f1a9cefde5b8",
    "embedding": null,
    "metadata": {
      "dataset_id": "3b3da4dc-09d2-43bc-8b46-b1a990c040ff",
      "account_id": "14fe321d-6768-4d9d-bbcf-d81a0a7fa51d",
      "file_id": "bb3c585c-18ca-4b45-97b6-e95b43145428",
      "file_name": "test.csv",
      "ref_doc_id": "bb3c585c-18ca-4b45-97b6-e95b43145428",
      "doc_id": "bb3c585c-18ca-4b45-97b6-e95b43145428"
    },
    "excluded_embed_metadata_keys": [],
    "excluded_llm_metadata_keys": [],
    "relationships": {},
    "text": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
    "start_char_idx": null,
    "end_char_idx": null,
    "text_template": "{metadata_str}\n\n{content}",
    "metadata_template": "{key}: {value}",
    "metadata_seperator": "\n",
    "class_name": "Document"
  },
  "__type__": "4"
}

When I query nodes, I am able to filter by fields within the metadata field. I want to create an index on the metadata field to speed up retrieval.

I want to add an _initialize function in the MongoDBKVStore to create an index on the metadata field, similar to how it is done in the PostgresKVStore, but in the _initialize function, the task is to create the index.

Reason

Indexes support efficient execution of queries in MongoDB. Without indexes, MongoDB must scan every document in a collection to return query results. If an appropriate index exists for a query, MongoDB uses the index to limit the number of documents it must scan.

Value of Feature

No response