config: add customization options for logging (#4383)

* config: add customization options for logging

* config: validate log fields

* allocate slices once

internal/log/access.go

@@ -0,0 +1,73 @@
+package log
+
+import (
+	"errors"
+	"fmt"
+)
+
+// An AccessLogField is a field in the access logs.
+type AccessLogField string
+
+// known access log fields
+const (
+	AccessLogFieldAuthority           AccessLogField = "authority"
+	AccessLogFieldDuration            AccessLogField = "duration"
+	AccessLogFieldForwardedFor        AccessLogField = "forwarded-for"
+	AccessLogFieldMethod              AccessLogField = "method"
+	AccessLogFieldPath                AccessLogField = "path"
+	AccessLogFieldReferer             AccessLogField = "referer"
+	AccessLogFieldRequestID           AccessLogField = "request-id"
+	AccessLogFieldResponseCode        AccessLogField = "response-code"
+	AccessLogFieldResponseCodeDetails AccessLogField = "response-code-details"
+	AccessLogFieldSize                AccessLogField = "size"
+	AccessLogFieldUpstreamCluster     AccessLogField = "upstream-cluster"
+	AccessLogFieldUserAgent           AccessLogField = "user-agent"
+)
+
+var defaultAccessLogFields = []AccessLogField{
+	AccessLogFieldUpstreamCluster,
+	AccessLogFieldMethod,
+	AccessLogFieldAuthority,
+	AccessLogFieldPath,
+	AccessLogFieldUserAgent,
+	AccessLogFieldReferer,
+	AccessLogFieldForwardedFor,
+	AccessLogFieldRequestID,
+	AccessLogFieldDuration,
+	AccessLogFieldSize,
+	AccessLogFieldResponseCode,
+	AccessLogFieldResponseCodeDetails,
+}
+
+// DefaultAccessLogFields returns the default access log fields.
+func DefaultAccessLogFields() []AccessLogField {
+	return defaultAccessLogFields
+}
+
+// ErrUnknownAccessLogField indicates that an access log field is unknown.
+var ErrUnknownAccessLogField = errors.New("unknown access log field")
+
+var accessLogFieldLookup = map[AccessLogField]struct{}{
+	AccessLogFieldAuthority:           {},
+	AccessLogFieldDuration:            {},
+	AccessLogFieldForwardedFor:        {},
+	AccessLogFieldMethod:              {},
+	AccessLogFieldPath:                {},
+	AccessLogFieldReferer:             {},
+	AccessLogFieldRequestID:           {},
+	AccessLogFieldResponseCode:        {},
+	AccessLogFieldResponseCodeDetails: {},
+	AccessLogFieldSize:                {},
+	AccessLogFieldUpstreamCluster:     {},
+	AccessLogFieldUserAgent:           {},
+}
+
+// Validate returns an error if the access log field is invalid.
+func (field AccessLogField) Validate() error {
+	_, ok := accessLogFieldLookup[field]
+	if !ok {
+		return fmt.Errorf("%w: %s", ErrUnknownAccessLogField, field)
+	}
+
+	return nil
+}