Add hints for promlog

Signed-off-by: Luca Comellini <luca.com@gmail.com>

promlog/flag/flag.go

@@ -14,6 +14,8 @@
 package flag
 
 import (
+	"strings"
+
 	kingpin "github.com/alecthomas/kingpin/v2"
 	"github.com/prometheus/common/promlog"
 )
@@ -23,23 +25,25 @@ import (
 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]"
+var LevelFlagHelp = "Only log messages with the given severity or above. One of: [" + strings.Join(promlog.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.
-const FormatFlagHelp = "Output format of log messages. One of: [logfmt, json]"
+var FormatFlagHelp = "Output format of log messages. One of: [" + strings.Join(promlog.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 *promlog.Config) {
 	config.Level = &promlog.AllowedLevel{}
 	a.Flag(LevelFlagName, LevelFlagHelp).
-		Default("info").SetValue(config.Level)
+		Default("info").HintOptions(promlog.LevelFlagOptions...).
+		SetValue(config.Level)
 
 	config.Format = &promlog.AllowedFormat{}
 	a.Flag(FormatFlagName, FormatFlagHelp).
-		Default("logfmt").SetValue(config.Format)
+		Default("logfmt").HintOptions(promlog.FormatFlagOptions...).
+		SetValue(config.Format)
 }

promlog/log.go

@@ -26,14 +26,17 @@ import (
 	"github.com/go-kit/log/level"
 )
 
+// This timestamp format differs from RFC3339Nano by using .000 instead
+// of .999999999 which changes the timestamp from 9 variable to 3 fixed
+// decimals (.130 instead of .130987456).
 var (
-	// This timestamp format differs from RFC3339Nano by using .000 instead
-	// of .999999999 which changes the timestamp from 9 variable to 3 fixed
-	// decimals (.130 instead of .130987456).
 	timestampFormat = log.TimestampFormat(
 		func() time.Time { return time.Now().UTC() },
 		"2006-01-02T15:04:05.000Z07:00",
 	)
+
+	LevelFlagOptions  = []string{"debug", "info", "warn", "error"}
+	FormatFlagOptions = []string{"logfmt, json"}
 )
 
 // AllowedLevel is a settable identifier for the minimum level a log entry