fix: proper error when parsing telemetry configuration (#12981)

When parsing `telemetry.global-labels` config the code assumes that the type will be an array.  I saw an issue where someone edited the configuration in the wrong way and got the following error:
![photo_2022-08-21_08-02-21](https://user-images.githubusercontent.com/22855163/185793842-c5759a54-1860-4dd1-bdb4-b94f4dab3c16.jpg)
Instead, I suggest here to print a proper error log to indicate what the issue is.

server/config/config.go

@@ -287,11 +287,18 @@ func DefaultConfig() *Config {
 }
 
 // GetConfig returns a fully parsed Config object.
-func GetConfig(v *viper.Viper) Config {
-	globalLabelsRaw := v.Get("telemetry.global-labels").([]interface{})
+func GetConfig(v *viper.Viper) (Config, error) {
+	globalLabelsRaw, ok := v.Get("telemetry.global-labels").([]interface{})
+	if !ok {
+		return Config{}, fmt.Errorf("failed to parse global-labels config")
+	}
+
 	globalLabels := make([][]string, 0, len(globalLabelsRaw))
-	for _, glr := range globalLabelsRaw {
-		labelsRaw := glr.([]interface{})
+	for idx, glr := range globalLabelsRaw {
+		labelsRaw, ok := glr.([]interface{})
+		if !ok {
+			return Config{}, fmt.Errorf("failed to parse global label number %d from config", idx)
+		}
 		if len(labelsRaw) == 2 {
 			globalLabels = append(globalLabels, []string{labelsRaw[0].(string), labelsRaw[1].(string)})
 		}
@@ -356,7 +363,7 @@ func GetConfig(v *viper.Viper) Config {
 			SnapshotInterval:   v.GetUint64("state-sync.snapshot-interval"),
 			SnapshotKeepRecent: v.GetUint32("state-sync.snapshot-keep-recent"),
 		},
-	}
+	}, nil
 }
 
 // ValidateBasic returns an error if min-gas-prices field is empty in BaseConfig. Otherwise, it returns nil.

server/start.go

@@ -208,7 +208,13 @@ func startStandAlone(ctx *Context, appCreator types.AppCreator) error {
 	}
 
 	app := appCreator(ctx.Logger, db, traceWriter, ctx.Viper)
-	_, err = startTelemetry(serverconfig.GetConfig(ctx.Viper))
+
+	config, err := serverconfig.GetConfig(ctx.Viper)
+	if err != nil {
+		return err
+	}
+
+	_, err = startTelemetry(config)
 	if err != nil {
 		return err
 	}
@@ -271,7 +277,11 @@ func startInProcess(ctx *Context, clientCtx client.Context, appCreator types.App
 		return err
 	}
 
-	config := serverconfig.GetConfig(ctx.Viper)
+	config, err := serverconfig.GetConfig(ctx.Viper)
+	if err != nil {
+		return err
+	}
+
 	if err := config.ValidateBasic(); err != nil {
 		return err
 	}