proxy: add support for logging http request headers

internal/log/access.go

@@ -3,6 +3,7 @@ package log
 import (
 	"errors"
 	"fmt"
+	"strings"
 )
 
 // An AccessLogField is a field in the access logs.
@@ -42,6 +43,21 @@ func DefaultAccessLogFields() []AccessLogField {
 	}
 }
 
+const accessLogFieldHeaderPrefix = "header."
+
+// AccessLogFieldForHeader returns an access log field for the given header name.
+func AccessLogFieldForHeader(header string) AccessLogField {
+	return AccessLogField(accessLogFieldHeaderPrefix + header)
+}
+
+// IsForHeader returns true if the access log field is for a header.
+func (field AccessLogField) IsForHeader() (headerName string, ok bool) {
+	if strings.HasPrefix(string(field), accessLogFieldHeaderPrefix) {
+		return string(field[len(accessLogFieldHeaderPrefix):]), true
+	}
+	return "", false
+}
+
 // ErrUnknownAccessLogField indicates that an access log field is unknown.
 var ErrUnknownAccessLogField = errors.New("unknown access log field")
 
@@ -62,6 +78,10 @@ var accessLogFieldLookup = map[AccessLogField]struct{}{
 
 // Validate returns an error if the access log field is invalid.
 func (field AccessLogField) Validate() error {
+	if _, ok := field.IsForHeader(); ok {
+		return nil
+	}
+
 	_, ok := accessLogFieldLookup[field]
 	if !ok {
 		return fmt.Errorf("%w: %s", ErrUnknownAccessLogField, field)