fix ts with no trailing zeros

Signed-off-by: Chao Chen <chaochn@amazon.com>

client/pkg/logutil/zap.go

@@ -58,7 +58,7 @@ var DefaultZapLoggerConfig = zap.Config{
 
 		// Custom EncodeTime function to ensure we match format and precision of historic capnslog timestamps
 		EncodeTime: func(t time.Time, enc zapcore.PrimitiveArrayEncoder) {
-			enc.AppendString(t.Format("2006-01-02T15:04:05.999999Z0700"))
+			enc.AppendString(t.Format("2006-01-02T15:04:05.000000Z0700"))
 		},
 
 		EncodeDuration: zapcore.StringDurationEncoder,

client/pkg/logutil/zap_test.go

@@ -0,0 +1,58 @@
+// Copyright 2024 The etcd 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 logutil
+
+import (
+	"bytes"
+	"encoding/json"
+	"regexp"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+)
+
+type commonLogFields struct {
+	Level     string `json:"level"`
+	Timestamp string `json:"ts"`
+	Message   string `json:"msg"`
+}
+
+const (
+	fractionSecondsPrecision = 6 // MicroSeconds
+)
+
+func TestEncodeTimePrecisionToMicroSeconds(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	syncer := zapcore.AddSync(buf)
+	zc := zapcore.NewCore(
+		zapcore.NewJSONEncoder(DefaultZapLoggerConfig.EncoderConfig),
+		syncer,
+		zap.NewAtomicLevelAt(zap.InfoLevel),
+	)
+
+	lg := zap.New(zc, zap.AddCaller())
+	lg.Info("TestZapLog")
+	fields := commonLogFields{}
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &fields))
+	// example 1: 2024-06-06T23:37:21.948385Z
+	// example 2 with zone offset: 2024-06-06T16:16:44.176778-0700
+	regex := `\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.(\d+)(Z|[+-]\d{4})`
+	re := regexp.MustCompile(regex)
+	matches := re.FindStringSubmatch(fields.Timestamp)
+	require.Equal(t, 3, len(matches))
+	require.Equalf(t, fractionSecondsPrecision, len(matches[1]), "unexpected timestamp %s", fields.Timestamp)
+}