Feat: automatically reference transaction from context on slog

I would like to introduce a handler that takes new relic transaction data from context automatically.

It is quite a common pattern to inject the base slog logger instance to some piece of code only so to pass it to nrslog.WithContext to receive a handler that captures logs in context. I find this api to be a bit leaky and introducing noise. Especially that slog already has methods which make context available for handlers.

With introduction of this handler one now can set a slog instance relying on a NRHandler as default one and call it from anywhere, ensuring that log calls with be passed to go-agent, as well as if the context contains transaction it will be linked. This is quite the case for http services which rely on newrelic.WrapHandle

an example:

on main.go

app, _ := newrelic.NewApplication(
  newrelic.ConfigAppName("foo"),
  newrelic.ConfigLicense("bar"),
)

instrumentedHandler := nrslog.WithTransactionFromContext(nrslog.TextHandler(app, os.Stderr, &slog.HandlerOptions{}))
slog.SetDefault(slog.New(instrumentedHandler))

on service.go

func DoSomeWork(ctx context.Context, app *newrelic.Application){
  txn := app.StartTransaction("baz")
  defer txn.End()

  ctx = newrelic.NewContext(ctx, txn)

  slog.InfoContext(ctx, "qux")  
}

All feedback is welcome. The implementation itself was hacked out pretty quick. I am open for suggestions for taking a different approach such as re-implementing the handler. At the moment this new feature is implemented using a decorator which disables setting the transactions externally and only allows passing transaction via context.

I hope this will serve as a good starting point for a discussion.

btw, not sure if the tests will pass as I found no way to run tests on local env.

newrelic / go-agent

Feat: automatically reference transaction from context on slog #880