feat(cli): Add Validate cli command to "test" the configuration (#964)

* feat(cli): Add Validate cli command to "test" the configuration

* update readme

README.md

@@ -268,14 +268,16 @@ USAGE:
    gitlab-ci-pipelines-exporter [global options] command [command options] [arguments...]
 
 COMMANDS:
-   run      start the exporter
-   monitor  display information about the currently running exporter
-   help, h  Shows a list of commands or help for one command
+   run       start the exporter
+   validate  validate the configuration file
+   monitor   display information about the currently running exporter
+   help, h   Shows a list of commands or help for one command
 
 GLOBAL OPTIONS:
    --internal-monitoring-listener-address value, -m value  internal monitoring listener address [$GCPE_INTERNAL_MONITORING_LISTENER_ADDRESS]
-   --help, -h                                              show help (default: false)
-   --version, -v                                           print the version (default: false)
+   --help, -h                                              show help
+   --version, -v                                           print the version
+
 ```
 
 ### run
@@ -296,6 +298,21 @@ OPTIONS:
    --help, -h                    show help (default: false)
 ```
 
+### validate
+
+```bash
+~$ gitlab-ci-pipelines-exporter validate --help
+NAME:
+   gitlab-ci-pipelines-exporter validate - validate the configuration file
+
+USAGE:
+   gitlab-ci-pipelines-exporter validate [command options] [arguments...]
+
+OPTIONS:
+   --config file, -c file  config file (default: "./gitlab-ci-pipelines-exporter.yml") [$GCPE_CONFIG]
+   --help, -h              show help
+```
+
 ### monitor
 
 ```bash

internal/cli/cli.go

@@ -71,6 +71,20 @@ func NewApp(version string, start time.Time) (app *cli.App) {
 				},
 			},
 		},
+		{
+			Name:   "validate",
+			Usage:  "validate the configuration file",
+			Action: cmd.ExecWrapper(cmd.Validate),
+			Flags: cli.FlagsByName{
+				&cli.StringFlag{
+					Name:    "config",
+					Aliases: []string{"c"},
+					EnvVars: []string{"GCPE_CONFIG"},
+					Usage:   "config `file`",
+					Value:   "./gitlab-ci-pipelines-exporter.yml",
+				},
+			},
+		},
 		{
 			Name:   "monitor",
 			Usage:  "display information about the currently running exporter",

internal/cmd/validate.go

@@ -0,0 +1,18 @@
+package cmd
+
+import (
+	log "github.com/sirupsen/logrus"
+	"github.com/urfave/cli/v2"
+)
+
+func Validate(cliCtx *cli.Context) (int, error) {
+	log.Debug("Validating configuration..")
+
+	if _, err := configure(cliCtx); err != nil {
+		log.WithError(err).Error("Failed to configure")
+		return 1, err
+	}
+
+	log.Debug("Configuration is valid")
+	return 0, nil
+}