Zap

package main

import (
  "time"

  "go.uber.org/zap"
)

func main() {
  logger := zap.NewExample()
  defer logger.Sync()

  url := "http://example.org/api"
  logger.Info("failed to fetch URL",
    zap.String("url", url),
    zap.Int("attempt", 3),
    zap.Duration("backoff", time.Second),
  )

  sugar := logger.Sugar()
  sugar.Infow("failed to fetch URL",
    "url", url,
    "attempt", 3,
    "backoff", time.Second,
  )
  sugar.Infof("Failed to fetch URL: %s", url)
}

{"level":"info","msg":"failed to fetch URL","url":"http://example.org/api","attempt":3,"backoff":"1s"}
{"level":"info","msg":"failed to fetch URL","url":"http://example.org/api","attempt":3,"backoff":"1s"}
{"level":"info","msg":"Failed to fetch URL: http://example.org/api"}