The Alexa APIs for NodeJS consists of JS and Typescript definitions that represent the request and response JSON of Alexa services. These models act as core dependency for the Alexa Skills Kit NodeJS SDK (https://github.com/alexa/alexa-skills-kit-sdk-for-nodejs).
SMAPI client supports the older and very limited Metrics API v1, but it doesn't support the newest and improved Metrics API v2.
See https://github.com/alexa/alexa-skills-kit-sdk-for-nodejs/issues/739