feat: new promslog/flag package to compliment promslog

Prereq for prometheus/prometheus#14355

This adds a new `promslog/flag` package to be used with the new promslog
package. It uses the same flag structure for a transition that is
hopefully easy for end users.

Signed-off-by: TJ Hoplock <[email protected]>

promlog/flag/flag.go

@@ -11,6 +11,11 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package flag defines standardised flag interactions for use with promlog
+// across Prometheus components.
+// It should typically only ever be imported by main packages.
+
+// Deprecated: This package has been replaced with github.com/prometheus/common/promslog/flag.
 package flag
 
 import (

promslog/flag/flag.go

@@ -0,0 +1,54 @@
+// Copyright 2024 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 defines standardised flag interactions for use with promslog
+// across Prometheus components.
+// It should typically only ever be imported by main packages.
+
+package flag
+
+import (
+	"strings"
+
+	kingpin "github.com/alecthomas/kingpin/v2"
+
+	"github.com/prometheus/common/promslog"
+)
+
+// 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.
+var LevelFlagHelp = "Only log messages with the given severity or above. One of: [" + strings.Join(promslog.LevelFlagOptions, ", ") + "]"
+
+// FormatFlagName is the canonical flag name to configure the log format
+// within Prometheus projects.
+const FormatFlagName = "log.format"
+
+// FormatFlagHelp is the help description for the log.format flag.
+var FormatFlagHelp = "Output format of log messages. One of: [" + strings.Join(promslog.FormatFlagOptions, ", ") + "]"
+
+// 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, config *promslog.Config) {
+	config.Level = &promslog.AllowedLevel{}
+	a.Flag(LevelFlagName, LevelFlagHelp).
+		Default("info").HintOptions(promslog.LevelFlagOptions...).
+		SetValue(config.Level)
+
+	config.Format = &promslog.AllowedFormat{}
+	a.Flag(FormatFlagName, FormatFlagHelp).
+		Default("logfmt").HintOptions(promslog.FormatFlagOptions...).
+		SetValue(config.Format)
+}