Adding support of log middleware (similar to rewriters) that supports promises

Please tell us about your environment:

• winston version?
  • winston@2
  • winston@3
• node -v outputs: 10.5.0
• Operating System? macOS Sierra
• Language? ES6/7

What’s the feature?

Adding support to add data to the log befora actually logging it (on the transport level). For example, if I use 2 transport - Console and Loggly - the console logs might be plain showing just the log message sent to it but I might want to add some additional information to the logs being sent to Loggly. I can create an async function to retreive the stack trace (using a package like stacktrace-js) and append it to the json being sent to Loggly.

This is just one example I believe it could help with.

[indexzero]

This is exactly what formats are for, but you are correct they are synchronous. Adding async support for formats is not currently planned. If enough folks 👍 this PR then we’ll consider reopening it.

Thanks for using winston!

[indexzero]

This is a long standing feature request tracked as https://github.com/winstonjs/winston/issues/200