Add distanceType support to ChromaVectorStore.Builder for distance function configuration

[TTzvet]

Expected Behavior

The ChromaVectorStore.Builder should support configuring the distance function, consistent with other Spring AI vector stores:

@Bean
public ChromaVectorStore vectorStore(EmbeddingModel embeddingModel, ChromaApi chromaApi) {
    return ChromaVectorStore.builder(chromaApi, embeddingModel)
        .collectionName("my-collection")
        .distanceType(ChromaDistanceType.COSINE)
        .initializeSchema(true)
        .build();
}

This would be consistent with other Spring AI vector stores that expose distance type configuration:

• PgVectorStore.Builder.distanceType()
• MariaDBVectorStore.Builder.distanceType()

Internally, Spring AI would translate this to ChromaDB's hnsw:space collection metadata.

Additionally, a generic collectionMetadata(Map<String, Object>) method could be useful for advanced ChromaDB-specific HNSW settings (e.g., hnsw:M, hnsw:construction_ef).

Current Behavior

The ChromaVectorStore.Builder does not expose any way to configure collection metadata. The builder only supports:

• collectionName()
• tenantName()
• databaseName()
• initializeSchema()
• filterExpressionConverter()
• initializeImmediately()

ChromaDB defaults to L2 (Euclidean distance), but for text/semantic search, cosine similarity is generally preferred. ChromaDB supports this via collection metadata (hnsw:space), but there's no way to pass this through Spring AI.

Context

I'm building a RAG application for semantic search over text documents. Cosine similarity is the recommended distance metric for text embeddings, but I cannot configure this through the ChromaVectorStore.Builder.

Workarounds considered:

1. Pre-create the collection in ChromaDB with the correct metadata before the application starts, then set initializeSchema(false) - this adds operational complexity and breaks the self-contained setup
2. Use ChromaApi directly to create the collection with metadata before building the VectorStore - this bypasses the builder pattern and duplicates configuration

Both workarounds are cumbersome compared to a simple builder method.

Environment:

• Spring AI Version: 1.0.2
• ChromaDB Version: 1.0.0