all: do not print usage info for all the flags when incorrect command-line flag is passed

This should improve usability for VictoriaMetrics apps that have big number of command-line flags,
i.e. all the apps.

app/vmagent/main.go

@@ -23,6 +23,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/cgroup"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envflag"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/flagutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver"
 	graphiteserver "github.com/VictoriaMetrics/VictoriaMetrics/lib/ingestserver/graphite"
 	influxserver "github.com/VictoriaMetrics/VictoriaMetrics/lib/ingestserver/influx"
@@ -275,8 +276,5 @@ vmagent collects metrics data via popular data ingestion protocols and routes it
 
 See the docs at https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/app/vmagent/README.md .
 `
-
-	f := flag.CommandLine.Output()
-	fmt.Fprintf(f, "%s\n", s)
-	flag.PrintDefaults()
+	flagutil.Usage(s)
 }

app/vmalert/main.go

@@ -219,8 +219,5 @@ vmalert processes alerts and recording rules.
 
 See the docs at https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/app/vmalert/README.md .
 `
-
-	f := flag.CommandLine.Output()
-	fmt.Fprintf(f, "%s\n", s)
-	flag.PrintDefaults()
+	flagutil.Usage(s)
 }

app/vmauth/main.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"flag"
-	"fmt"
 	"net/http"
 	"net/http/httputil"
 	"net/url"
@@ -12,6 +11,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/cgroup"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envflag"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/flagutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/procutil"
@@ -100,8 +100,5 @@ vmauth authenticates and authorizes incoming requests and proxies them to Victor
 
 See the docs at https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/app/vmauth/README.md .
 `
-
-	f := flag.CommandLine.Output()
-	fmt.Fprintf(f, "%s\n", s)
-	flag.PrintDefaults()
+	flagutil.Usage(s)
 }

app/vmbackup/main.go

@@ -103,10 +103,7 @@ or local filesystem. Backed up data can be restored with vmrestore.
 
 See the docs at https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/app/vmbackup/README.md .
 `
-
-	f := flag.CommandLine.Output()
-	fmt.Fprintf(f, "%s\n", s)
-	flag.PrintDefaults()
+	flagutil.Usage(s)
 }
 
 func newSrcFS() (*fslocal.FS, error) {

app/vmrestore/main.go

@@ -62,10 +62,7 @@ vmrestore restores VictoriaMetrics data from backups made by vmbackup.
 
 See the docs at https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/app/vmrestore/README.md .
 `
-
-	f := flag.CommandLine.Output()
-	fmt.Fprintf(f, "%s\n", s)
-	flag.PrintDefaults()
+	flagutil.Usage(s)
 }
 
 func newDstFS() (*fslocal.FS, error) {

docs/CHANGELOG.md

@@ -5,6 +5,8 @@
 * FEATURE: optimize Consul service discovery speed when discovering big number of services. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/574
 * FEATURE: add `label_uppercase(q, label1, ... labelN)` and `label_lowercase(q, label1, ... labelN)` function to [MetricsQL](https://victoriametrics.github.io/MetricsQL.html)
   for uppercasing and lowercasing values for the given labels. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/936
+* FEATURE: do not print usage info for all the command-line flags when incorrect command-line flag is passed. Previously it could be hard reading the error message
+  about incorrect command-line flag because of too big usage info for all the flags.
 
 * BUGFIX: properly parse timestamps in OpenMetrics format - they are exposed as floating-point number in seconds instead of integer milliseconds
   unlike in Prometheus exposition format. See [the docs](https://github.com/OpenObservability/OpenMetrics/blob/master/specification/OpenMetrics.md#timestamps).

lib/flagutil/usage.go

@@ -0,0 +1,39 @@
+package flagutil
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Usage prints s and optional description for all the flags if -h or -help flag is passed to the app.
+func Usage(s string) {
+	f := flag.CommandLine.Output()
+	fmt.Fprintf(f, "%s\n", s)
+	if hasHelpFlag(os.Args[1:]) {
+		flag.PrintDefaults()
+	} else {
+		fmt.Fprintf(f, `Run "%s -help" in order to see the description for all the available flags`+"\n", os.Args[0])
+	}
+}
+
+func hasHelpFlag(args []string) bool {
+	for _, arg := range args {
+		if isHelpArg(arg) {
+			return true
+		}
+	}
+	return false
+}
+
+func isHelpArg(arg string) bool {
+	if !strings.HasPrefix(arg, "-") {
+		return false
+	}
+	arg = arg[1:]
+	if strings.HasPrefix(arg, "-") {
+		arg = arg[1:]
+	}
+	return arg == "h" || arg == "help"
+}