funcr: add example

This shows what output looks like and how call depth is handled.

funcr/example/main.go

@@ -0,0 +1,56 @@
+/*
+Copyright 2021 The logr Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/go-logr/logr"
+	"github.com/go-logr/logr/funcr"
+)
+
+type E struct {
+	str string
+}
+
+func (e E) Error() string {
+	return e.str
+}
+
+func Helper(log logr.Logger, msg string) {
+	helper2(log, msg)
+}
+
+func helper2(log logr.Logger, msg string) {
+	log.WithCallDepth(2).Info(msg)
+}
+
+func main() {
+	log := funcr.New(
+		func(pfx, args string) { fmt.Println(pfx, args) },
+		funcr.Options{LogCaller: funcr.All})
+	example(log.WithValues("module", "example"))
+}
+
+func example(log logr.Logger) {
+	log.Info("hello", "val1", 1, "val2", map[string]int{"k": 1})
+	log.V(1).Info("you should see this")
+	log.V(1).V(1).Info("you should NOT see this")
+	log.Error(nil, "uh oh", "trouble", true, "reasons", []float64{0.1, 0.11, 3.14})
+	log.Error(E{"an error occurred"}, "goodbye", "code", -1)
+	Helper(log, "thru a helper")
+}