feat: add structured log writer

[garethgeorge]

Background

When using a standard go logger e.g. "log" package's log.Default() we don't get a "spanId" / "trace" in the logs generated:

// HTTP is a simple HTTP function that writes the request body to the response body.
func HTTPNoLog(w http.ResponseWriter, r *http.Request) {
	log.Default().Println("handling HTTP request without span / trace ID logging")
	body, err := ioutil.ReadAll(r.Body)
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		return
	}
	if err := ioutil.WriteFile(outputFile, body, 0644); err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		return
	}
}

This can make it hard to associate logs printed with the invocation generating them e.g.

No span ID / trace ID is captured for logs generated by default when using concurrency.

Proposal

Include a funcframework package utility for creating io.Writers that produce structured logs and that are compatible with common logging frameworks (e.g. "log" by default.

// HTTP is a simple HTTP function that writes the request body to the response body.
func HTTP(w http.ResponseWriter, r *http.Request) {
	l := log.New(funcframework.LogWriter(r.Context()), "", log.Lshortfile)
	l.Println("handling HTTP request")
	body, err := ioutil.ReadAll(r.Body)
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		return
	}
	if err := ioutil.WriteFile(outputFile, body, 0644); err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		return
	}
}

Or, alternatively a CX can extract the trace ID and other logging IDs from the request context if needed using the utilities:

funcframework.ExecutionIDFromContext(ctx context.Context)
funcframework.TraceIDFromContext(ctx context.Context)
funcframework.SpanIDFromContext(ctx context.Context)