Don't nest attributes under message in newrelic logs appender

[fabn]

Environment

Provide at least:

• Ruby Version: 3.2.2
• Semantic Logger Version: 4.16.0
• Application/framework names and versions (Rails 7.x)
• Rails Semantic Logger Version, if applicable. 4.17.0

Expected Behavior

In JSON should be something like:

{
  "entity.guid": "MTQ0ODQ2MHxBUE18QVBQTElDQVRJT058OTY2Mjk2ODk",
  "entity.guids": "MTQ0ODQ2MHxBUE18QVBQTElDQVRJT058OTY2Mjk2ODk",
  "entity.name": "Motohelp Rails (staging)",
  "entity.type": "SERVICE",
  "file.name": "(pry)",
  "hostname": "worker-6b6c685bf7-8b8c8",
  "level": "INFO",
  "line.number": "2",
  "logger.name": "Test",
  "application": "Semantic Logger",
  "duration": 100,
  "duration_human": "100.0ms",
  "environment": "production",
  "message": "Login time",
  "payload.ip_address": "127.0.0.1",
  "payload.user": "Joe",
  "tags": "[]",
  "newrelic.source": "logs.APM",
  "thread.name": "6500",
  "timestamp": 1727874680026
}

Actual Behavior

Current behavior implemented in #222 is to nest everything under message key (I guess because NR method to log doesn't support attributes), so by using your appender we get this:

It would be super nice to lift everything in top level, They implemented newrelic/newrelic-ruby-agent#2084 (and also have a proposal to have them contextually newrelic/newrelic-ruby-agent#2297)

I don't know if NewRelic::Agent.agent.log_event_aggregator.add_custom_attributes(foo: 'bar', baz: 'bat') can be used as is, but it could help.