fix(middleware-logger): remove $metadata from response.output

[trivikr]

Issue #, if available:
Fixes: #1490

Description of changes:
remove $metadata from response.output

Test code

const { DynamoDB } = require("../aws-sdk-js-v3/clients/client-dynamodb");

(async () => {
  const region = "us-east-1";
  const client = new DynamoDB({ region });

  console.log(await client.listTables({ Limit: 1 }));
})();

Output before

{
  '$metadata': {
    httpStatusCode: 200,
    httpHeaders: {
      server: 'Server',
      date: 'Mon, 14 Dec 2020 20:40:57 GMT',
      'content-type': 'application/x-amz-json-1.0',
      'content-length': '44',
      connection: 'keep-alive',
      'x-amzn-requestid': 'M7HOMU8M8R280SCB6QD266MOC3VV4KQNSO5AEMVJF66Q9ASUAAJG',
      'x-amz-crc32': '2729330041'
    },
    requestId: 'M7HOMU8M8R280SCB6QD266MOC3VV4KQNSO5AEMVJF66Q9ASUAAJG',
    attempts: 1,
    totalRetryDelay: 0
  },
  LastEvaluatedTableName: undefined,
  TableNames: [ 'aws-js-sdk-workshop-notes' ]
}

Output after

{
  LastEvaluatedTableName: undefined,
  TableNames: [ 'aws-js-sdk-workshop-notes' ]
}

The metadata is logged by middleware-logger in #1788

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[trivikr]

Converting to draft as making $metadata optional is a big change and would require updates/modifications in integ tests.

[trivikr]

During post-scrum on 12/18, we decided not to remove metadata as we're already GA and it's easy to find important information from $metadata.
We'll remove httpHeaders from metadata though in #1491

[github-actions]

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