example.rb

#!/usr/bin/env ruby

require_relative './kv_logger.rb'

logger = KVLogger.new(STDOUT)
logger.info "sup"

logger = logger.with_tags(tag: 'info')

logger.warn "with tags!"
logger.untagged.warn "without!"

logger.info "it's fine\nwith newline"

class ComplexThing
  attr_accessor :foo, :bar

  def initialize(foo, bar)
    self.foo = foo
    self.bar = bar
  end

  def to_json(*)
    {
      foo: self.foo,
      bar: self.bar,
    }.to_json
  end
end

logger = logger.with_tags(complex: ComplexThing.new("baz", "qux"))
logger.info "works with random objects!"

logger = logger.with_tags(random_garbage: binding)
logger.info "won't barf!"

kv_logger.rb

require 'logger'
require 'json'

class KVLogger < Logger
  class JSONFormatter
    attr_accessor :tags

    def initialize(tags = {})
      self.tags = tags
    end

    def call(severity, time, progname, message)
      JSON.generate(
        {
          severity: severity,
          time: time,
          progname: progname,
          message: message,
          tags: tags
        }
      ) + "\n"
    end
  end

  def initialize(*args, **kwargs)
    super

    self.formatter = JSONFormatter.new
  end

  def with_tags(tags)
    new_logger = self.clone
    new_logger.formatter = JSONFormatter.new(self.formatter.tags.merge(tags))

    new_logger
  end

  def untagged
    new_logger = self.clone
    new_logger.formatter = JSONFormatter.new

    new_logger
  end
end

output.txt

 [fg386-server] kv_logger > ./example.rb
{"severity":"INFO","time":"2023-02-18 20:40:07 -0600","progname":null,"message":"sup","tags":{}}
{"severity":"WARN","time":"2023-02-18 20:40:07 -0600","progname":null,"message":"with tags!","tags":{"tag":"info"}}
{"severity":"WARN","time":"2023-02-18 20:40:07 -0600","progname":null,"message":"without!","tags":{}}
{"severity":"INFO","time":"2023-02-18 20:40:07 -0600","progname":null,"message":"it's fine\nwith newline","tags":{"tag":"info"}}
{"severity":"INFO","time":"2023-02-18 20:40:07 -0600","progname":null,"message":"works with random objects!","tags":{"tag":"info","complex":{"foo":"baz","bar":"qux"}}}
{"severity":"INFO","time":"2023-02-18 20:40:07 -0600","progname":null,"message":"won't barf!","tags":{"tag":"info","complex":{"foo":"baz","bar":"qux"},"random_garbage":"#<Binding:0x000055955a2b2768>"}}