[autodiscovery] Update Config File Reader to Support Discovery Probe (#52087)

### What does this PR do?

Allow the config file reader to process configs with only a Discovery attribute defined. This is for a new feature that will not be released yet.

Note, we also don't want to report the integration.Config to the backend because it still needs to be fully discovered at runtime in Autodiscovery using the discovery probes that will be implemented. Currently the raw file provided config looks like:

```
ad_identifiers:
  -  krakend
discovery: {}
```

but with the Discover feature it will later be converted into something that is actually scheduled like:

```
ad_identifiers:
  -  krakend
discovery: {}
instances:
  - openmetrics_url: localhost:1312/telemetry
```

### Describe how you validated your changes

Unit Tests and CI

Co-authored-by: gabe.dossantos <gabe.dossantos@datadoghq.com>

Author: gabedos

comp/core/autodiscovery/configresolver/configresolver.go

@@ -48,6 +48,7 @@ func Resolve(tpl integration.Config, svc listeners.Service) (integration.Config,
 		MetricsExcluded: svc.HasFilter(filter.MetricsFilter),
 		LogsExcluded:    svc.HasFilter(filter.LogsFilter),
 		ImageName:       svc.GetImageName(),
+		Discovery:       tpl.Discovery,
 	}
 	copy(resolvedConfig.InitConfig, tpl.InitConfig)
 	copy(resolvedConfig.Instances, tpl.Instances)

comp/core/autodiscovery/configresolver/configresolver_test.go

@@ -907,6 +907,26 @@ func TestResolve(t *testing.T) {
 				ServiceID:     "a5901276aed1",
 			},
 		},
+		{
+			testName: "discovery marker is preserved through resolution",
+			svc: &dummyService{
+				ID:            "a5901276aed1",
+				ADIdentifiers: []string{"redis"},
+				Hosts:         map[string]string{"bridge": "127.0.0.1"},
+			},
+			tpl: integration.Config{
+				Name:          "redis",
+				ADIdentifiers: []string{"redis"},
+				Discovery:     &integration.DiscoveryConfig{},
+			},
+			out: integration.Config{
+				Name:          "redis",
+				ADIdentifiers: []string{"redis"},
+				Instances:     []integration.Data{},
+				Discovery:     &integration.DiscoveryConfig{},
+				ServiceID:     "a5901276aed1",
+			},
+		},
 	}
 
 	for i, tc := range testCases {

comp/core/autodiscovery/providers/config_reader.go

@@ -51,6 +51,7 @@ type ConfigFormatWrapper struct {
 	ConfigFormat string
 	Filename     string
 	Hash         string
+	IsDiscovery  bool // Marks this config as a discovery template (instances will be populated at runtime later)
 }
 
 type configPkg struct {
@@ -445,9 +446,9 @@ func GetIntegrationConfigFromFile(name, fpath string) (integration.Config, Confi
 		return conf, ConfigFormatWrapper{}, err
 	}
 
-	// If no valid instances were found & this is neither a metrics file, nor a logs file
-	// this is not a valid configuration file
-	if cf.MetricConfig == nil && cf.LogsConfig == nil && len(cf.Instances) < 1 {
+	// If no valid instances were found & this is neither a metrics file, a logs file,
+	// nor a discovery-only template, this is not a valid configuration file
+	if cf.MetricConfig == nil && cf.LogsConfig == nil && cf.Discovery == nil && len(cf.Instances) < 1 {
 		return conf, ConfigFormatWrapper{}, errors.New("Configuration file contains no valid instances")
 	}
 
@@ -523,7 +524,7 @@ func GetIntegrationConfigFromFile(name, fpath string) (integration.Config, Confi
 	conf.Source = "file:" + fpath
 	hash := sha256.Sum256([]byte(scrubbedConfigFormat))
 
-	return conf, ConfigFormatWrapper{ConfigFormat: scrubbedConfigFormat, Filename: fpath, Hash: hex.EncodeToString(hash[:])}, err
+	return conf, ConfigFormatWrapper{ConfigFormat: scrubbedConfigFormat, Filename: fpath, Hash: hex.EncodeToString(hash[:]), IsDiscovery: cf.Discovery != nil}, err
 }
 
 func containsString(slice []string, str string) bool {

comp/core/autodiscovery/providers/config_reader_test.go

@@ -129,6 +129,14 @@ func TestGetIntegrationConfig(t *testing.T) {
 	config, _, err = GetIntegrationConfigFromFile("foo", "tests/testcheck.yaml")
 	require.Nil(t, err)
 	assert.Nil(t, config.Discovery)
+
+	// discovery-only: a file with only `discovery: {}` is valid and has no instances.
+	config, _, err = GetIntegrationConfigFromFile("foo", "tests/discovery_only.yaml")
+	require.Nil(t, err)
+	require.NotNil(t, config.Discovery, "discovery-only file should produce a non-nil Discovery field")
+	assert.Empty(t, config.Instances)
+	assert.Nil(t, config.MetricConfig)
+	assert.Nil(t, config.LogsConfig)
 }
 
 func TestReadConfigFiles(t *testing.T) {
@@ -137,7 +145,7 @@ func TestReadConfigFiles(t *testing.T) {
 
 	configs, errors, err := ReadConfigFiles(GetAll)
 	require.Nil(t, err)
-	require.Equal(t, 22, len(configs))
+	require.Equal(t, 23, len(configs))
 	require.Equal(t, 4, len(errors))
 
 	for _, c := range configs {
@@ -148,7 +156,7 @@ func TestReadConfigFiles(t *testing.T) {
 
 	configs, _, err = ReadConfigFiles(WithoutAdvancedAD)
 	require.Nil(t, err)
-	require.Equal(t, 20, len(configs))
+	require.Equal(t, 21, len(configs))
 
 	expectedConfig1 := integration.Config{
 		Name: "advanced_ad",

comp/core/autodiscovery/providers/file_test.go

@@ -82,7 +82,7 @@ func TestCollect(t *testing.T) {
 	assert.Equal(t, 0, len(get("ignored")))
 
 	// total number of configurations found
-	assert.Equal(t, 19, len(configs))
+	assert.Equal(t, 20, len(configs))
 
 	// incorrect configs get saved in the Errors map (invalid.yaml & notaconfig.yaml & ad_deprecated.yaml & null_instances.yml)
 	assert.Equal(t, 4, len(provider.Errors))

comp/core/autodiscovery/providers/tests/discovery_only.yaml

@@ -0,0 +1,4 @@
+ad_identifiers:
+  - discovery_only_id
+
+discovery: {}