Add promlog package.

promlog/flag/flag.go

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+// Copyright 2017 The Prometheus 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 flag
+
+import (
+	"github.com/prometheus/common/promlog"
+	kingpin "gopkg.in/alecthomas/kingpin.v2"
+)
+
+// LevelFlagName is the canonical flag name to configure the allowed log level
+// within Prometheus projects.
+const LevelFlagName = "log.level"
+
+// LevelFlagHelp is the help description for the log.level flag.
+const LevelFlagHelp = "Only log messages with the given severity or above. One of: [debug, info, warn, error]"
+
+// AddFlags adds the flags used by this package to the Kingpin application.
+// To use the default Kingpin application, call AddFlags(kingpin.CommandLine)
+func AddFlags(a *kingpin.Application, logLevel *promlog.AllowedLevel) {
+	a.Flag(LevelFlagName, LevelFlagHelp).
+		Default("info").SetValue(logLevel)
+}

promlog/log.go

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+// Copyright 2017 The Prometheus 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 promlog defines standardised ways to initialize Go kit loggers
+// across Prometheus components.
+// It should typically only ever be imported by main packages.
+package promlog
+
+import (
+	"os"
+
+	"github.com/go-kit/kit/log"
+	"github.com/go-kit/kit/log/level"
+	"github.com/pkg/errors"
+)
+
+// AllowedLevel is a settable identifier for the minimum level a log entry
+// must be have.
+type AllowedLevel struct {
+	s string
+	o level.Option
+}
+
+func (l *AllowedLevel) String() string {
+	return l.s
+}
+
+// Set updates the value of the allowed level.
+func (l *AllowedLevel) Set(s string) error {
+	switch s {
+	case "debug":
+		l.o = level.AllowDebug()
+	case "info":
+		l.o = level.AllowInfo()
+	case "warn":
+		l.o = level.AllowWarn()
+	case "error":
+		l.o = level.AllowError()
+	default:
+		return errors.Errorf("unrecognized log level %q", s)
+	}
+	l.s = s
+	return nil
+}
+
+// New returns a new leveled oklog logger in the logfmt format. Each logged line will be annotated
+// with a timestamp. The output always goes to stderr.
+func New(al AllowedLevel) log.Logger {
+	l := log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
+	l = log.With(l, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)
+	return l
+}