Monitoring Log Volume - `go-kit/log`

// log/fmt.go
package log

import (
	"sync"
	"bytes"
	"io"

	"github.com/go-logfmt/logfmt"
	"github.com/go-kit/log/level"
	logkit "github.com/go-kit/log"
)

type logfmtEncoder struct {
	*logfmt.Encoder
	buf bytes.Buffer
}

func (l *logfmtEncoder) Reset() {
	l.Encoder.Reset()
	l.buf.Reset()
}

var logfmtEncoderPool = sync.Pool{
	New: func() interface{} {
		var enc logfmtEncoder
		enc.Encoder = logfmt.NewEncoder(&enc.buf)
		return &enc
	},
}

type logfmtLogger struct {
	w io.Writer
}

func NewLogfmtLogger(w io.Writer) logkit.Logger {
	return &logfmtLogger{w}
}

func (l logfmtLogger) Log(keyvals ...interface{}) error {
	enc := logfmtEncoderPool.Get().(*logfmtEncoder)
	enc.Reset()
	defer logfmtEncoderPool.Put(enc)

	if err := enc.EncodeKeyvals(keyvals...); err != nil {
		return err
	}

	if err := enc.EndRecord(); err != nil {
		return err
	}

	n, err := l.w.Write(enc.buf.Bytes())

	lev := "unknown"
	for i := 1; i < len(keyvals); i += 2 {
		if v, ok := keyvals[i].(level.Value); ok {
			lev = v.String()
			break
		}
	}

	logMessages.WithLabelValues(lev).Inc()
	logMessagesSize.WithLabelValues(lev).Add(float64(n))

	return err
}

// log/log.go
package log

import (
	"os"

	logkit "github.com/go-kit/log"
	"github.com/go-kit/log/level"

	"github.com/prometheus/client_golang/prometheus"
)

var (
	logMessages = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "log_messages_total",
		Help: "Total number of log messages.",
	}, []string{"level"})

	logMessagesSize = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "log_messages_size_total",
		Help: "Total size of log messages.",
	}, []string{"level"})

	supportedLevels = []level.Value{
		level.DebugValue(),
		level.InfoValue(),
		level.WarnValue(),
		level.ErrorValue(),
	}
)

func init() {
	prometheus.MustRegister(logMessages)
	prometheus.MustRegister(logMessagesSize)
}

type InstrumentedLogger struct {
	logger logkit.Logger
}

func NewInstrumentedLogger(options level.Value, keyvals ...interface{}) (logkit.Logger, error) {
	logger := NewLogfmtLogger(logkit.NewSyncWriter(os.Stderr))

	logger = logkit.With(logger, "ts", logkit.DefaultTimestampUTC)
	logger = logkit.With(logger, keyvals...)
	logger = level.NewFilter(logger, level.Allow(options))

	for _, level := range supportedLevels {
		logMessages.WithLabelValues(level.String())
		logMessagesSize.WithLabelValues(level.String())
	}

	return &InstrumentedLogger{
		logger: logger,
	}, nil
}

func (pl *InstrumentedLogger) Log(kv ...interface{}) error {
	pl.logger.Log(kv...)
	return nil
}

// main.go
package main

import (
	"net/http"

	"github.com/mircodz/log/log"

	"github.com/go-kit/log/level"

	"github.com/prometheus/client_golang/prometheus/promhttp"
)

func main() {
	logger, err := log.NewInstrumentedLogger(level.DebugValue())
	if err != nil {
		panic(err)
	}

	level.Debug(logger).Log("message", "Hello, World!")
	level.Info(logger).Log("message", "Hello, World!")
	level.Warn(logger).Log("message", "Hello, World!")
	level.Error(logger).Log("message", "Hello, World!")

	http.Handle("/metrics", promhttp.Handler())
    http.ListenAndServe(":2112", nil)
}