Structured logging

[haikoschol]

It would be useful to have structured logging for all messages produced by the node binary (probably opt-in via command line flag). #3260 mentions structured logging, but only in the context of telemetry.
Right now we rely on logstash to turn plain text logs into JSON, which sometimes gets it wrong. A concrete example is a debug message triggering an alert, probably because it contained the string "Error".

[kianenigma]

Just randomly pinging @sorpaas because of his recent work on logging. Maybe it will be helpful? #6825

[tomaka]

For what it's worth, the printing is currently done here and here.

It should be quite easy IMO to add a CLI flag that sets the kind of output.