logging: Implement log_append handler (#6066)

* logging: Implement `extra_log` handler

* Rename to `log_append`

* Rename `skip_log` to `log_skip`

---------

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/logging/caddyfile.go

@@ -0,0 +1,53 @@
+// Copyright 2015 Matthew Holt and The Caddy 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 logging
+
+import (
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+)
+
+func init() {
+	httpcaddyfile.RegisterHandlerDirective("log_append", parseCaddyfile)
+}
+
+// parseCaddyfile sets up the log_append handler from Caddyfile tokens. Syntax:
+//
+//	log_append [<matcher>] <key> <value>
+func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
+	handler := new(LogAppend)
+	err := handler.UnmarshalCaddyfile(h.Dispenser)
+	return handler, err
+}
+
+// UnmarshalCaddyfile implements caddyfile.Unmarshaler.
+func (h *LogAppend) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	d.Next() // consume directive name
+	if !d.NextArg() {
+		return d.ArgErr()
+	}
+	h.Key = d.Val()
+	if !d.NextArg() {
+		return d.ArgErr()
+	}
+	h.Value = d.Val()
+	return nil
+}
+
+// Interface guards
+var (
+	_ caddyfile.Unmarshaler = (*LogAppend)(nil)
+)