TypeScript code:

function myMetricsHandler(metrics) {
  // metrics is provided by the function that calls myMetricsHandler,
  // in this case "metricScope"
  return async function butts(input) {
    return metrics.someChainedFuncsThatPublishAMetric(input.exampleKey);
  }
}

const myPublish = metricScope(myMetricsHandler);
await myPublish({exampleKey: 5})


// Where in the AWS library, metricScope might be defined as something like:
function metricScope(userFunction) {
  const metrics = new MetricsService();
  return async function (args) {
    // first, get the async function returned by myMetricsHandler
    const buttsFunction = userFunction(metrics);
    // then call that async function with the final arguments ({exampleKey: 5}).
    // butts will have access to metrics by virtue of being defined inside
    // the closure created by myMetricsHandler
    return buttsFunction(args);
  }
}