是否可以打印错误？

For example such code:

os.Stderr.WriteString(rec.(string))

But this will not show as an error:

I know that I can panic after logging and catch it on API Gateway (against sending stacktrace to the client) - no other ways? Documentation is not mention anything like that.

It seems not possible. I assume, you're looking at the metrics in Amazon CloudWatch

AWS Lambda automatically monitors functions on your behalf, reporting metrics through Amazon CloudWatch. These metrics include total invocations, errors, duration, throttles, DLQ errors and Iterator age for stream-based invocations. https://docs.aws.amazon.com/lambda/latest/dg/monitoring-functions-metrics.html

Now, let's see how do they define errors

Metric "Errors" measures the number of invocations that failed due to errors in the function (response code 4XX).

So, if you want to see the errors on that graph, you have to respond with the proper codes. If you're concerned about exposing the error stacktrace, here is a good read Error handling with API Gateway and Go Lambda functions. The basic idea there is about creating a custom lambdaError type, meant to be used by a Lambda handler function to wrap errors before returning them. This custom error message

{
  "code": "TASK_NOT_FOUND",
  "public_message": "Task not found",
  "private_message": "unknown task: foo-bar"
}

will be wrapped in a standard one

{
  "errorMessage": "{\"code\":\"TASK_NOT_FOUND\",\"public_message\":\"Task not found\",\"private_message\":\"unknown task: foo-bar\"}",
  "errorType": "lambdaError"
}

and later on mapped in API Gateway, so, the end client will see only the public message

{
  "code": "TASK_NOT_FOUND",
  "message": "Task not found"
}