Winston / Stackdriver severity level

winston-stackdriver.js

const winston = require('winston');
const { LEVEL } = require('triple-beam');

const SeverityLookup = {
  'default': 'DEFAULT',
  'silly': 'DEFAULT',
  'verbose': 'DEBUG',
  'debug': 'DEBUG',
  'http': 'notice',
  'info': 'info',
  'warn': 'WARNING',
  'error': 'ERROR',
}

const stackdriverSeverityFormat = winston.format((info) => ({
  ...info,
  // Add severity to your log
  severity: SeverityLookup[info[LEVEL]] || SeverityLookup['default'],
}));

const formatters = [
  winston.format.timestamp(),
  // Add the format that supplements the JSON with severity
  stackdriverSeverityFormat(),
  winston.format.json(),
];

const logger = winston.createLogger({
  level: 'silly',
  format: winston.format.combine(...formatters),
  stderrLevels: ['error'],
  transports: [
    new winston.transports.Console(),
  ],
});

Thank you @jasperkuperus this has been very helpful !
just in case someone wants to go a step further and preserve the errors in the logged output, here's how we went about it :

function preserveErrors(input) {
  const output = {};
  Object.entries(input).forEach(([key, value]) => {
    let outputValue = value;
    if (value instanceof Error) {
      outputValue = {};
      Object.getOwnPropertyNames(value).forEach((errorKey) => {
        outputValue[errorKey] = value[errorKey];
      });
    }
    if (Object.prototype.toString.call(value) === '[object Object]') {
      outputValue = preserveErrors(value);
    }

    output[key] = outputValue;
  });
  return output;
}
const SeverityLookup = {
    default: 'DEFAULT',
    silly: 'DEFAULT',
    verbose: 'DEBUG',
    debug: 'DEBUG',
    http: 'notice',
    info: 'info',
    warn: 'WARNING',
    error: 'ERROR',
  };

  // add severity level for GCP stackdriver
  // cf https://github.com/googleapis/nodejs-logging-winston/issues/386#issuecomment-602195498
  const stackdriverSeverityFormat = format((info) => ({
    ...info,
    severity: SeverityLookup[info[LEVEL]] || SeverityLookup.default,
  }));

  const errorsPreserverFormat = format((info) => {
    // we spread the original `info` too in order to preserve the keys behind Symbols (i.e. splat/level)
    return {
      ...info,
      ...preserveErrors(info),
    };
  });

  const formatters = [format.timestamp(), stackdriverSeverityFormat(), errorsPreserverFormat(), format.json()];

  const loggingWinston = new transports.Console({
    format: format.combine(...formatters),
    stderrLevels: ['error'],
  });

It's probably not bulletproof, but it can help too.
We get decent output in GCP with severity and errors, and that is what we wanted from the nodejs-logging-winston lib. It probably offers more features for error handling on the GCP side, but we don't miss it for now
At least now the logs are found when clicking on "instance logs" of a deployment in GCP, which is a breeze :)