proxy: add support for logging http request headers (#4388)

* config: add customization options for logging

* config: validate log fields

* proxy: add support for logging http request headers

* log subset of headers

* fix test name

* dont use log.HTTPHeaders for access logs

* canonicalize http/2 headers

internal/log/headers_test.go

@@ -0,0 +1,63 @@
+package log
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/rs/zerolog"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestHTTPHeaders(t *testing.T) {
+	t.Parallel()
+
+	type A = []string
+	type M = map[string]string
+
+	t.Run("all", func(t *testing.T) {
+		t.Parallel()
+
+		var buf bytes.Buffer
+		log := zerolog.New(&buf)
+		evt := log.Info()
+		evt = HTTPHeaders(evt, A{"headers"}, M{
+			"a": "1",
+			"b": "2",
+			"c": "3",
+		})
+		evt.Send()
+
+		assert.Equal(t, `{"level":"info","headers":{"A":"1","B":"2","C":"3"}}`, strings.TrimSpace(buf.String()))
+	})
+	t.Run("none", func(t *testing.T) {
+		t.Parallel()
+
+		var buf bytes.Buffer
+		log := zerolog.New(&buf)
+		evt := log.Info()
+		evt = HTTPHeaders(evt, A{"a", "b", "c"}, M{
+			"a": "1",
+			"b": "2",
+			"c": "3",
+		})
+		evt.Send()
+
+		assert.Equal(t, `{"level":"info"}`, strings.TrimSpace(buf.String()))
+	})
+	t.Run("two", func(t *testing.T) {
+		t.Parallel()
+
+		var buf bytes.Buffer
+		log := zerolog.New(&buf)
+		evt := log.Info()
+		evt = HTTPHeaders(evt, A{"headers.a", "headers.C"}, M{
+			"a": "1",
+			"b": "2",
+			"c": "3",
+		})
+		evt.Send()
+
+		assert.Equal(t, `{"level":"info","headers":{"A":"1","C":"3"}}`, strings.TrimSpace(buf.String()))
+	})
+}