Marshal server group inline configs.

Currently, the /config page doesn't show service discovery
configuration, since it's an inline configuration and ignored by the
default yaml marshaller. This patch uses the MarshalYAMLWithInlineConfigs
helper from prometheus to include the service discovery configuration in the
marshalled yaml.

pkg/servergroup/config.go

@@ -209,6 +209,11 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	return nil
 }
 
+// MarshalYAML implements the yaml.Marshaler interface.
+func (c *Config) MarshalYAML() (interface{}, error) {
+	return discovery.MarshalYAMLWithInlineConfigs(c)
+}
+
 // HTTPClientConfig extends prometheus' HTTPClientConfig
 type HTTPClientConfig struct {
 	DialTimeout time.Duration                `yaml:"dial_timeout"`