internal/telemetry: add an example of using the logging behaviour

And also fix that the output was not actually correct!

Change-Id: If81e22e586b1c2a71c69843b49d5dd8d5d2dfde6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227298
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

internal/telemetry/event/event.go

@@ -70,6 +70,11 @@ func (ev Event) Format(f fmt.State, r rune) {
 	}
 	for it := ev.Tags(); it.Valid(); it.Advance() {
 		tag := it.Tag()
+		// msg and err were both already printed above, so we skip them to avoid
+		// double printing
+		if tag.Key == Msg || tag.Key == Err {
+			continue
+		}
 		fmt.Fprintf(f, "\n\t%v", tag)
 	}
 }

internal/telemetry/export/log_test.go

@@ -0,0 +1,37 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package export_test
+
+import (
+	"context"
+	"errors"
+	"os"
+	"time"
+
+	"golang.org/x/tools/internal/telemetry/event"
+	"golang.org/x/tools/internal/telemetry/export"
+)
+
+func ExampleLog() {
+	ctx := context.Background()
+	event.SetExporter(timeFixer(export.LogWriter(os.Stdout, false)))
+	anInt := event.NewIntKey("myInt", "an integer")
+	aString := event.NewStringKey("myString", "a string")
+	event.Print(ctx, "my event", anInt.Of(6))
+	event.Error(ctx, "error event", errors.New("an error"), aString.Of("some string value"))
+	// Output:
+	// 2020/03/05 14:27:48 my event
+	// 	myInt=6
+	// 2020/03/05 14:27:48 error event: an error
+	// 	myString="some string value"
+}
+
+func timeFixer(output event.Exporter) event.Exporter {
+	at, _ := time.Parse(time.RFC3339Nano, "2020-03-05T14:27:48Z")
+	return func(ctx context.Context, ev event.Event, tagMap event.TagMap) context.Context {
+		ev.At = at
+		return output(ctx, ev, tagMap)
+	}
+}