[service] Return telemetry.Config validation errors (#12100)

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

Currently these are just printed, which hides validation issues with
`telemetry.Config`. If we don't want to return these errors, we should
document that and print them at a warning log level.

.chloggen/service-config-validate.yaml

@@ -0,0 +1,25 @@
+# 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. otlpreceiver)
+component: service
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Include validation errors from telemetry.Config when validating the service config
+
+# One or more tracking issues or pull requests related to the change
+issues: [12100]
+
+# (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: Previously validation errors were only printed to the console
+
+# 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: []

otelcol/testdata/otelcol-emptyreaders.yaml

@@ -7,6 +7,7 @@ exporters:
 service:
   telemetry:
     metrics:
+      level: none
       readers: []
   pipelines:
     metrics:

service/config.go

@@ -29,7 +29,7 @@ func (cfg *Config) Validate() error {
 	}
 
 	if err := cfg.Telemetry.Validate(); err != nil {
-		fmt.Printf("service::telemetry config validation failed: %v\n", err)
+		return fmt.Errorf("service::telemetry config validation failed: %w", err)
 	}
 
 	return nil

service/config_test.go

@@ -71,14 +71,19 @@ func TestConfigValidate(t *testing.T) {
 				cfg.Telemetry.Metrics.Readers = nil
 				return cfg
 			},
-			expected: nil,
+			expected: errors.New("service::telemetry config validation failed: collector telemetry metrics reader should exist when metric level is not none"),
 		},
 	}
 
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {
 			cfg := tt.cfgFn()
-			assert.Equal(t, tt.expected, cfg.Validate())
+			err := cfg.Validate()
+			if tt.expected != nil {
+				assert.EqualError(t, err, tt.expected.Error())
+			} else {
+				assert.NoError(t, err)
+			}
 		})
 	}
 }