Add more explicit logging RE recording rules

Promxy doesn't currently support storing recording rules, because
prometheus has no remote write API -- meaning promxy has no place to put
the metrics. This used to log (before the 2.x upgrade)-- but that was
dropped and this is confusing again.

This patch adds 2 layers of logging. (1) in main it checks for recording
rules on start (where it will fatal log) and on reload (error log) (2)
the ProxyStorage.Appender() now also logs a warning. This will remain in
place until #79 (the long term plan) is figured out and executed

Fixes #74

cmd/promxy/main.go

@@ -306,6 +306,22 @@ func main() {
 		logrus.Fatalf("Error loading config: %s", err)
 	}
 
+	// Our own checks on top of the config
+	checkConfig := func() error {
+		// check for any recording rules, if we find any lets log a fatal and stop
+		// https://github.com/jacksontj/promxy/issues/74
+		for _, rule := range ruleManager.Rules() {
+			if _, ok := rule.(*rules.RecordingRule); ok {
+				return fmt.Errorf("Promxy doesn't support recording rules: %s", rule)
+			}
+		}
+		return nil
+	}
+
+	if err := checkConfig(); err != nil {
+		logrus.Fatalf("Error checking config: %v", err)
+	}
+
 	close(reloadReady)
 
 	// Set up access logger
@@ -349,6 +365,9 @@ func main() {
 				if err := reloadConfig(reloadables...); err != nil {
 					log.Errorf("Error reloading config: %s", err)
 				}
+				if err := checkConfig(); err != nil {
+					logrus.Errorf("Error checking config: %v", err)
+				}
 			case syscall.SIGTERM, syscall.SIGINT:
 				log.Infof("promxy exiting")
 				cancel()

proxystorage/proxy.go

@@ -120,6 +120,7 @@ func (a *appenderStub) Commit() error { return nil }
 func (a *appenderStub) Rollback() error { return nil }
 
 func (p *ProxyStorage) Appender() (storage.Appender, error) {
+	logrus.Warning("Promxy cannot *write* metrics but is being asked to. This is likely due to a RecordingRule")
 	return &appenderStub{}, nil
 }