LIGHTSAIL SD: Validate HTTP config

Related-to #12810

Signed-off-by: Jayapriya Pai <janantha@redhat.com>

config/config_test.go

@@ -1749,6 +1749,10 @@ var expectedErrors = []struct {
 		filename: "ec2_token_file.bad.yml",
 		errMsg:   `at most one of bearer_token & bearer_token_file must be configured`,
 	},
+	{
+		filename: "lightsail_token_file.bad.yml",
+		errMsg:   `at most one of bearer_token & bearer_token_file must be configured`,
+	},
 	{
 		filename: "section_key_dup.bad.yml",
 		errMsg:   "field scrape_configs already set in type config.plain",

config/testdata/lightsail_token_file.bad.yml

@@ -0,0 +1,6 @@
+scrape_configs:
+  - job_name: foo
+    lightsail_sd_configs:
+      - region: us-east-1
+        bearer_token: foo
+        bearer_token_file: foo

discovery/aws/lightsail.go

@@ -109,7 +109,7 @@ func (c *LightsailSDConfig) UnmarshalYAML(unmarshal func(interface{}) error) err
 		}
 		c.Region = region
 	}
-	return nil
+	return c.HTTPClientConfig.Validate()
 }
 
 // LightsailDiscovery periodically performs Lightsail-SD requests. It implements