fix(inputs/kube_inventory): set TLS server name config properly

The TLS config currently always sets the ServerName to the same as the baseURL. A URL is not a valid value, e.g. `https://kubernetes.example.com:6443` is not a valid ServerName value; the matching ServerName value should be `kubernetes.example.com`.

This results in an error like -

```
2021-10-21T12:15:40Z E! [inputs.kube_inventory] Error in plugin: Get "https://kubernetes.example.com:6443/apis/apps/v1/namespaces/default/deployments": x509: certificate is valid for kubernetes, kubernetes.default, kubernetes.default.svc, kubernetes.default.svc.cluster.local, localhost, kubernetes.example.com, not https://kubernetes.example.com:6443
```

The ServerName value is not read from config, so I'm not sure why this currently works for anyone, unless they disable TLS verification through the `insecure_skip_verify` setting.

The fix is to set ServerName to the `tls_server_name` config value if set, otherwise leave it blank, in which case, it will automatically use the hostname (https://github.com/kubernetes/client-go/blob/master/rest/config.go#L219).

I can confirm that the inputs/kubernetes plugin does this (although it doesn't use the k8s client-go library), and it works properly by passing through the `tls_server_name` value or leaving it blank.

plugins/inputs/kube_inventory/README.md

@@ -74,6 +74,8 @@ avoid cardinality issues:
   # tls_cert = "/path/to/certfile"
   ## Used for TLS client certificate authentication
   # tls_key = "/path/to/keyfile"
+  ## Send the specified TLS server name via SNI
+  # tls_server_name = "kubernetes.example.com"
   ## Use TLS but skip chain & host verification
   # insecure_skip_verify = false
 

plugins/inputs/kube_inventory/client.go

@@ -23,7 +23,7 @@ type client struct {
 func newClient(baseURL, namespace, bearerToken string, timeout time.Duration, tlsConfig tls.ClientConfig) (*client, error) {
 	c, err := kubernetes.NewForConfig(&rest.Config{
 		TLSClientConfig: rest.TLSClientConfig{
-			ServerName: baseURL,
+			ServerName: tlsConfig.ServerName,
 			Insecure:   tlsConfig.InsecureSkipVerify,
 			CAFile:     tlsConfig.TLSCA,
 			CertFile:   tlsConfig.TLSCert,

plugins/inputs/kube_inventory/kube_state.go

@@ -80,6 +80,7 @@ var sampleConfig = `
   # tls_ca = "/path/to/cafile"
   # tls_cert = "/path/to/certfile"
   # tls_key = "/path/to/keyfile"
+  # tls_server_name = "kubernetes.example.com"
   ## Use TLS but skip chain & host verification
   # insecure_skip_verify = false
 `