Add SeverityUndefined to otel/log (#5072)

* Add SeverityUndefined

* Add changelog entry

---------

Co-authored-by: Sam Xie <sam@samxie.me>

log/severity.go

@@ -13,6 +13,9 @@ type Severity int
 
 // Severity values defined by OpenTelemetry.
 const (
+	// SeverityUndefined represents an unset Severity.
+	SeverityUndefined Severity = 0 // UNDEFINED
+
 	// A fine-grained debugging log record. Typically disabled in default
 	// configurations.
 	SeverityTrace1 Severity = 1 // TRACE

log/severity_string.go

@@ -8,6 +8,7 @@ func _() {
 	// An "invalid array index" compiler error signifies that the constant values have changed.
 	// Re-run the stringer command to generate them again.
 	var x [1]struct{}
+	_ = x[SeverityUndefined-0]
 	_ = x[SeverityTrace1-1]
 	_ = x[SeverityTrace2-2]
 	_ = x[SeverityTrace3-3]
@@ -34,14 +35,13 @@ func _() {
 	_ = x[SeverityFatal4-24]
 }
 
-const _Severity_name = "TRACETRACE2TRACE3TRACE4DEBUGDEBUG2DEBUG3DEBUG4INFOINFO2INFO3INFO4WARNWARN2WARN3WARN4ERRORERROR2ERROR3ERROR4FATALFATAL2FATAL3FATAL4"
+const _Severity_name = "UNDEFINEDTRACETRACE2TRACE3TRACE4DEBUGDEBUG2DEBUG3DEBUG4INFOINFO2INFO3INFO4WARNWARN2WARN3WARN4ERRORERROR2ERROR3ERROR4FATALFATAL2FATAL3FATAL4"
 
-var _Severity_index = [...]uint8{0, 5, 11, 17, 23, 28, 34, 40, 46, 50, 55, 60, 65, 69, 74, 79, 84, 89, 95, 101, 107, 112, 118, 124, 130}
+var _Severity_index = [...]uint8{0, 9, 14, 20, 26, 32, 37, 43, 49, 55, 59, 64, 69, 74, 78, 83, 88, 93, 98, 104, 110, 116, 121, 127, 133, 139}
 
 func (i Severity) String() string {
-	i -= 1
 	if i < 0 || i >= Severity(len(_Severity_index)-1) {
-		return "Severity(" + strconv.FormatInt(int64(i+1), 10) + ")"
+		return "Severity(" + strconv.FormatInt(int64(i), 10) + ")"
 	}
 	return _Severity_name[_Severity_index[i]:_Severity_index[i+1]]
 }

log/severity_test.go

@@ -19,6 +19,12 @@ func TestSeverity(t *testing.T) {
 		value    int
 		str      string
 	}{
+		{
+			name:     "SeverityUndefined",
+			severity: log.SeverityUndefined,
+			value:    0,
+			str:      "UNDEFINED",
+		},
 		{
 			name:     "SeverityTrace",
 			severity: log.SeverityTrace,