fix golint issues in core/prometheus (#506)

core/prometheus/agent.go

@@ -12,6 +12,7 @@ import (
 
 var once sync.Once
 
+// StartAgent starts a prometheus agent.
 func StartAgent(c Config) {
 	once.Do(func() {
 		if len(c.Host) == 0 {

core/prometheus/config.go

@@ -1,5 +1,6 @@
 package prometheus
 
+// A Config is a prometheus config.
 type Config struct {
 	Host string `json:",optional"`
 	Port int    `json:",default=9101"`