lib/promscrape: follow-up for 8537533beb

- Add a comment describing the purpose of the `role` field inside `apiConfig` struct
- Revert changes at lib/promscrape/discovery/dockerswarm/dockerswarm.go ,
  since they reduce code readability. E.g. the reader needs to look up the named string constants
  in order to get their values.

lib/promscrape/discovery/dockerswarm/api.go

@@ -17,6 +17,8 @@ type apiConfig struct {
 	port   int
 
 	// role is the type of objects to discover.
+	//
+	// filtersQueryArg is applied only to the given role - the rest of objects are queried without filters.
 	role string
 
 	// filtersQueryArg contains escaped `filters` query arg to add to each request to Docker Swarm API.

lib/promscrape/discovery/dockerswarm/dockerswarm.go

@@ -14,12 +14,6 @@ var SDCheckInterval = flag.Duration("promscrape.dockerswarmSDCheckInterval", 30*
 	"This works only if dockerswarm_sd_configs is configured in '-promscrape.config' file. "+
 	"See https://prometheus.io/docs/prometheus/latest/configuration/configuration/#dockerswarm_sd_config for details")
 
-const (
-	roleTasks    = "tasks"
-	roleServices = "services"
-	roleNodes    = "nodes"
-)
-
 // SDConfig represents docker swarm service discovery configuration
 //
 // See https://prometheus.io/docs/prometheus/latest/configuration/configuration/#dockerswarm_sd_config
@@ -48,11 +42,11 @@ func (sdc *SDConfig) GetLabels(baseDir string) ([]map[string]string, error) {
 		return nil, fmt.Errorf("cannot get API config: %w", err)
 	}
 	switch sdc.Role {
-	case roleTasks:
+	case "tasks":
 		return getTasksLabels(cfg)
-	case roleServices:
+	case "services":
 		return getServicesLabels(cfg)
-	case roleNodes:
+	case "nodes":
 		return getNodesLabels(cfg)
 	default:
 		return nil, fmt.Errorf("unexpected `role`: %q; must be one of `tasks`, `services` or `nodes`; skipping it", sdc.Role)