Custom DynamoDB Client in @langchain/community/stores/message/dynamodb

[alexbaileyuk]

Checked

• I searched existing ideas and did not find a similar one
• I added a very descriptive title
• I've clearly described the feature request and motivation for it

Feature request

As a user of the @langchain/community/stores/message/dynamodb component, I'd like to be able to provide my own DynamoDBClient rather than it being created for me in the constructor.

Motivation

I would like to be able to instrument the client with AWS X-Ray. It would also allow me to instantiate the client once and re-use the client across other areas of my application.

Proposal (If applicable)

From what I can tell, the constructor could be changed with relative ease:

  export interface DynamoDBChatMessageHistoryFields {
    tableName: string;
    sessionId: string;
    partitionKey?: string;
    sortKey?: string;
    messageAttributeName?: string;
    config?: DynamoDBClientConfig;
+  client?: DynamoDBClient;
    key?: Record<string, AttributeValue>;
  }

...

  constructor({
    tableName,
    sessionId,
    partitionKey,
    sortKey,
    messageAttributeName,
    config,
+  client,
    key = {},
  }: DynamoDBChatMessageHistoryFields) {
    super();

+ const clientConfig = config ?? {};
+ this.client = client ?? new DynamoDBClient(clientConfig);

    this.tableName = tableName;
    this.sessionId = sessionId;
-  this.client = new DynamoDBClient(config ?? {});
    this.partitionKey = partitionKey ?? this.partitionKey;
    this.sortKey = sortKey;
    this.messageAttributeName =
      messageAttributeName ?? this.messageAttributeName;
    this.dynamoKey = key;

    // override dynamoKey with partition key and sort key when key not specified
    if (Object.keys(this.dynamoKey).length === 0) {
      this.dynamoKey[this.partitionKey] = { S: this.sessionId };
      if (this.sortKey) {
        this.dynamoKey[this.sortKey] = { S: this.sortKey };
      }
    }
  }

I'd be happy to take a look at this in a PR. Would just like some steer on how we'd want tests modifying. Would we want to duplicate all tests for both options (providing custom client vs providing client config) or should we just have a 'happy path' test for it?