Lambda Custom Integrations vs. Proxy Integration

https://github.com/elastic/apm-agent-nodejs/pull/2194#discussion_r742178899

> From @trentm
> https://docs.aws.amazon.com/lambda/latest/dg/services-apigateway.html#apigateway-example-event talks about an API > Gateway "custom integration" vs. a "proxy integration".
>> For a proxy integration, the event has a defined structure.
> Is our [spec](https://github.com/elastic/apm/blob/master/specs/agents/tracing-instrumentation-aws-lambda.md#api-gateway-v1--v2) and this code just about *proxy* integrations?
>What is a "custom integration"? https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-api-integration-types.html
>> In contrast, the Lambda custom integration allows for reuse of configured mapping templates for various integration endpoints that have similar requirements of the input and output data formats. The setup is more involved and is recommended for more advanced application scenarios.
> A comment added here, and possibly changing the name to `isApiGatewayProxyEvent()` would be helpful.
Perhaps "custom integrations" should be discussed separate from this PR.