add lint and fix some issue

.golangci.yml

@@ -0,0 +1,35 @@
+version: "2"
+run:
+  tests: true
+linters:
+  enable:
+    - govet
+    - ineffassign
+    - staticcheck
+    - unused
+    - bodyclose
+    - gocognit
+    - dupl
+    - errcheck
+    - modernize
+    - errname
+    - errorlint
+    - prealloc
+
+  settings:
+    gocognit:
+      min-complexity: 15
+    dupl:
+      threshold: 100
+    modernize:
+      disable:
+        - plusbuild
+    errcheck:
+      exclude-functions:
+        - fmt.Fprintf
+        - (io.Closer).Close
+        - (*os.File).Close
+        - (*net.TCPConn).Close
+        - (*net.TCPListener).Close
+        - (*bytes.Buffer).Write
+        - (*strings.Builder).WriteString

Makefile

@@ -15,3 +15,5 @@ check:
 	go test -v ./...
 	go test -race ./...
 
+lint:
+	golangci-lint run --timeout 5m ./...

statsfile.go

@@ -29,13 +29,13 @@ func writeStats(dir, filename string, st procfs.CPUStat) error {
 	err = je.Encode(stats{st, n})
 	if err != nil {
 		newFile.Close()
-		os.Remove(newFile.Name())
+		_ = os.Remove(newFile.Name())
 		return err
 	}
 
 	err = newFile.Close()
 	if err != nil {
-		os.Remove(newFile.Name())
+		_ = os.Remove(newFile.Name())
 		return err
 	}
 

usageplugin.go

@@ -131,11 +131,12 @@ func (u LinuxUsagePlugin) gaugeNetMetrics(pf procfs.FS) (map[string]float64, err
 	if err != nil {
 		return res, err
 	}
-	if psnmp.Tcp.ActiveOpens != nil {
-		res["active"] = *psnmp.Tcp.ActiveOpens
+	tcp := psnmp.Tcp
+	if tcp.ActiveOpens != nil {
+		res["active"] = *tcp.ActiveOpens
 	}
-	if psnmp.Tcp.PassiveOpens != nil {
-		res["passive"] = *psnmp.Tcp.PassiveOpens
+	if tcp.PassiveOpens != nil {
+		res["passive"] = *tcp.PassiveOpens
 	}
 	if pnetstat.ListenOverflows != nil {
 		res["overflows"] = *pnetstat.ListenOverflows