Add ability to include metadata to searchable documents for other vector search providers

[sergiobayona]

See: https://github.com/patterns-ai-core/langchainrb/issues/318 for context.

metadata feature was recently added for the pgvector provider. See https://github.com/patterns-ai-core/langchainrb/pull/859

Need to add the same feature to the rest of the supported vector search providers.