RFC: an objectMode stream implementation for winston Transports

[indexzero]

The winston@3 efforts have definitely crossed the 80% mark. This PR represents the final semantics for winston-transport base implementations:

• TransportStream: the new objectMode Writable stream which should be the base for all future Transports after winston >= 3.
• LegacyTransportStream: the backwards compatible wrap to Transports written for winston < 3. There isn't all that much different for those implementors except that log(level, message, meta, callback) is now log(info, callback) where info is the object being plumbed along the objectMode pipe-chain. This was absolutely critical to not "break the ecosystem" and give the over 500 Transport package authors an upgrade path.

Discuss

There are currently three outstanding items that warrant discussion:

Practical implications of implementing ._writev:

The current approach will be to make an optional .logv that TransportStream implementors can take advantage of if they wish. If .logv does not exist then ._writev will call .log multiple times.

Formats for TransportStream instances: winston@3

Moves all of the log formatting into user-land and performs some plumbing in .format where it reads from the underlying _format and ._transform where it writes to the _format. We need to do this same bookkeeping in TransportStream since there are obvious Transport-specific formatting choices (e.g. to colorize console output, but not file output).

Pipe from multiple parent Logger instances:

This will work no problem today, unless the parent Logger instances are configured with different levels. While this is technically possible from winston.Container code paths, it is an unknown if the user-base is utilizing it. If you share Transports across multiple Loggers WITH DIFFERENT LEVELS please chime in here. Your input is necessary as we are strongly considering dropping this feature.

Items for merge

• 100% code coverage for TransportStream
• 100% code coverage for LegacyTransportStream
• Implementation of _writev for TransportStream
• Implementation of _writev for LegacyTransportStream
• (Moved to a future PR) Implementation of format on TransportStream