Hide block duration flags. (prometheus#3618)

Users are starting to use these mistakenly thinking they'll help
with issues, and thus causing some confusion.
Thus hide them and make it clear that they're only there for testing
reasons.

cmd/prometheus/main.go

@@ -148,12 +148,12 @@ func main() {
 	a.Flag("storage.tsdb.path", "Base path for metrics storage.").
 		Default("data/").StringVar(&cfg.localStoragePath)
 
-	a.Flag("storage.tsdb.min-block-duration", "Minimum duration of a data block before being persisted.").
-		Default("2h").SetValue(&cfg.tsdb.MinBlockDuration)
+	a.Flag("storage.tsdb.min-block-duration", "Minimum duration of a data block before being persisted. For use in testing.").
+		Hidden().Default("2h").SetValue(&cfg.tsdb.MinBlockDuration)
 
 	a.Flag("storage.tsdb.max-block-duration",
-		"Maximum duration compacted blocks may span. (Defaults to 10% of the retention period)").
-		PlaceHolder("<duration>").SetValue(&cfg.tsdb.MaxBlockDuration)
+		"Maximum duration compacted blocks may span. For use in testing. (Defaults to 10% of the retention period).").
+		Hidden().PlaceHolder("<duration>").SetValue(&cfg.tsdb.MaxBlockDuration)
 
 	a.Flag("storage.tsdb.retention", "How long to retain samples in the storage.").
 		Default("15d").SetValue(&cfg.tsdb.Retention)