Nicer error message when passing an empty configuration file (#9762)

This PR checks if `cfg.Validate()` error is `errMissingReceivers` error
then returns a nicely formated error.

otelcol/config.go

@@ -12,8 +12,9 @@ import (
 )
 
 var (
-	errMissingExporters = errors.New("no exporter configuration specified in config")
-	errMissingReceivers = errors.New("no receiver configuration specified in config")
+	errMissingExporters       = errors.New("no exporter configuration specified in config")
+	errMissingReceivers       = errors.New("no receiver configuration specified in config")
+	errEmptyConfigurationFile = errors.New("empty configuration file")
 )
 
 // Config defines the configuration for the various elements of collector or agent.
@@ -42,6 +43,11 @@ type Config struct {
 // invalid cases that we currently don't check for but which we may want to add in
 // the future (e.g. disallowing receiving and exporting on the same endpoint).
 func (cfg *Config) Validate() error {
+	// There must be at least one property set in the configuration	file.
+	if len(cfg.Receivers) == 0 && len(cfg.Exporters) == 0 && len(cfg.Processors) == 0 && len(cfg.Connectors) == 0 && len(cfg.Extensions) == 0 {
+		return errEmptyConfigurationFile
+	}
+
 	// Currently, there is no default receiver enabled.
 	// The configuration must specify at least one receiver to be valid.
 	if len(cfg.Receivers) == 0 {

otelcol/config_test.go

@@ -54,6 +54,19 @@ func TestConfigValidate(t *testing.T) {
 			},
 			expected: nil,
 		},
+		{
+			name: "empty configuration file",
+			cfgFn: func() *Config {
+				cfg := generateConfig()
+				cfg.Receivers = nil
+				cfg.Connectors = nil
+				cfg.Processors = nil
+				cfg.Exporters = nil
+				cfg.Extensions = nil
+				return cfg
+			},
+			expected: errEmptyConfigurationFile,
+		},
 		{
 			name: "missing-exporters",
 			cfgFn: func() *Config {