Trident logs http: Server closed as a fatal error when it shutdowns

[tksm]

Describe the bug
Trident logs http: Server closed as a fatal error when it shutdowns.

time="2021-07-01T01:47:30Z" level=fatal msg="http: Server closed"

Tident components call func (*Server) Shutdown, so ErrServerClosed(http: Server closed) is returned in normal cases.

• https://github.com/NetApp/trident/blob/v21.04.1/frontend/metrics/plugin.go#L41-L43
• https://github.com/NetApp/trident/blob/v21.04.1/frontend/rest/apiserver_http.go#L43-L46
• https://github.com/NetApp/trident/blob/v21.04.1/frontend/rest/apiserver_https.go#L69-L72

Environment

• Trident version: 21.04.1
• Trident installation flags used: silenceAutosupport: true (Trident Operator)
• Container runtime: Docker 19.03.13
• Kubernetes version: v1.19.11
• Kubernetes orchestrator: Kubernetes
• Kubernetes enabled feature gates:
• OS: Ubuntu 18.04.5
• NetApp backend types: ONTAP AFF 9.1P14
• Other:

To Reproduce

1. Watch the log of one trident-csi pod
   • e.g. kubectl logs -n trident --since 10s -f trident-csi-xxxx
2. Delete the trident-csi pod
   • e.g. kubectl delete pod -n trident trident-csi-xxxx

Expected behavior
ErrServerClosed(http: Server closed) should be ignored like below. (ListenAndServe always returns a non-nil error.)

diff --git a/frontend/metrics/plugin.go b/frontend/metrics/plugin.go
index fd1562f..0c2a109 100644
--- a/frontend/metrics/plugin.go
+++ b/frontend/metrics/plugin.go
@@ -38,7 +38,7 @@ func (s *Server) Activate() error {
        go func() {
                log.WithField("address", s.server.Addr).Info("Activating metrics frontend.")
                http.Handle("/metrics", s.server.Handler)
-               if err := s.server.ListenAndServe(); err != nil {
+               if err := s.server.ListenAndServe(); err != http.ErrServerClosed {
                        log.Fatal(err)
                }
        }()

[gnarl]

This is fixed with commit ddddc0b and will be included in the Trident v21.10 release.