Properly handle default endpoint for annotation discovery (#40792)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Annotation discovery should not add the default endpoint explicitly.
Configuration
should be left empty if users do not set anything. Main flow of the
receiver_creator
already adds the default endpoint, if not provided, at a later stage by
also checking
if target receiver supports it. This patch ensures this logic is unified
for both
annotation discovery and templated based discovery.

Main flow of the receiver creator handles default endpoint at
https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/1ff442c73096b3770070d0fd8868d42467245134/receiver/receivercreator/observerhandler.go#L158-L163

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes
#40712

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Fixed

<!--Describe the documentation added.-->
#### Documentation
Not needed


/cc @dmitryax

---------

Signed-off-by: ChrsMark <[email protected]>
Co-authored-by: Dmitry Anoshin <[email protected]>

Author: ChrsMark

.chloggen/fix_discovery_default_endpoint.yaml

@@ -0,0 +1,32 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: receivercreator
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Properly handle default endpoint for annotation discovery
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [40712]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  Annotation discovery should not add the default endpoint explicitly. Configuration
+  should be left empty if users do not set anything. Main flow of the receiver_creator
+  already adds the default endpoint, if not provided, at a later stage by also checking
+  if target receiver supports it. This patch ensures this logic is unified for both
+  annotation discovery and templated based discovery.
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/receivercreator/discovery.go

@@ -180,18 +180,23 @@ func getScraperConfFromAnnotations(
 	defaultEndpoint, scopeSuffix string,
 	logger *zap.Logger,
 ) (userConfigMap, error) {
-	conf := userConfigMap{}
-	conf[endpointConfigKey] = defaultEndpoint
-
 	configStr, found := getHintAnnotation(annotations, otelMetricsHints, configHint, scopeSuffix)
 	if !found || configStr == "" {
-		return conf, nil
+		// defaultEndpoint will be added properly later in observerHandler.startReceiver method
+		return userConfigMap{}, nil
 	}
+	conf := userConfigMap{}
 	if err := yaml.Unmarshal([]byte(configStr), &conf); err != nil {
 		return userConfigMap{}, fmt.Errorf("could not unmarshal configuration from hint: %v", zap.Error(err))
 	}
 
-	val := conf[endpointConfigKey]
+	var val any
+	var endpointSet bool
+	if val, endpointSet = conf[endpointConfigKey]; !endpointSet {
+		// skip endpoint's validation if there is no user provided endpoint
+		// defaultEndpoint will be added properly later in observerHandler.startReceiver method
+		return conf, nil
+	}
 	confEndpoint, ok := val.(string)
 	if !ok {
 		logger.Debug("could not extract configured endpoint")

receiver/receivercreator/discovery_test.go

@@ -26,12 +26,14 @@ func TestK8sHintsBuilderMetrics(t *testing.T) {
 collection_interval: "20s"
 timeout: "30s"
 username: "username"
-password: "changeme"`
+password: "changeme"
+endpoint: 1.2.3.4:6379`
 	configRedis := `
 collection_interval: "20s"
 timeout: "130s"
 username: "username"
-password: "changeme"`
+password: "changeme"
+endpoint: 1.2.3.4:6379`
 
 	tests := map[string]struct {
 		inputEndpoint    observer.Endpoint
@@ -109,7 +111,7 @@ password: "changeme"`
 			expectedReceiver: receiverTemplate{
 				receiverConfig: receiverConfig{
 					id:     id,
-					config: userConfigMap{"endpoint": "1.2.3.4:6379"},
+					config: userConfigMap{},
 				}, signals: receiverSignals{metrics: true, logs: false, traces: false},
 			},
 			wantError:       false,
@@ -624,7 +626,6 @@ nested_example:
 				"io.opentelemetry.discovery.metrics/config":  configNoEndpoint,
 			}, expectedConf: userConfigMap{
 				"collection_interval": "20s",
-				"endpoint":            "1.1.1.1:8080",
 				"initial_delay":       "20s",
 				"read_buffer_size":    "10",
 				"nested_example":      userConfigMap{"foo": "bar"},