feat(outputs.nats): add nkey seed authentication support

plugins/outputs/nats/README.md

@@ -46,6 +46,9 @@ to use them.
   ## Optional NATS 2.0 and NATS NGS compatible user credentials
   # credentials = "/etc/telegraf/nats.creds"
 
+  ## Optional authentication with nkey seed file (NATS 2.0)
+  # nkey_seed = "/etc/telegraf/seed.txt"
+
   ## NATS subject for producer messages.
   ## This field can be a static string or a Go template, see README for details.
   ## Incompatible with `use_batch_format

plugins/outputs/nats/nats.go

@@ -31,6 +31,7 @@ type NATS struct {
 	Username       config.Secret `toml:"username"`
 	Password       config.Secret `toml:"password"`
 	Credentials    string        `toml:"credentials"`
+	NkeySeed       string        `toml:"nkey_seed"`
 	Subject        string        `toml:"subject"`
 	UseBatchFormat bool          `toml:"use_batch_format"`
 	Jetstream      *StreamConfig `toml:"jetstream"`
@@ -120,6 +121,14 @@ func (n *NATS) Connect() error {
 		opts = append(opts, nats.UserCredentials(n.Credentials))
 	}
 
+	if n.NkeySeed != "" {
+		opt, err := nats.NkeyOptionFromSeed(n.NkeySeed)
+		if err != nil {
+			return err
+		}
+		opts = append(opts, opt)
+	}
+
 	if n.Name != "" {
 		opts = append(opts, nats.Name(n.Name))
 	}

plugins/outputs/nats/sample.conf

@@ -13,6 +13,9 @@
   ## Optional NATS 2.0 and NATS NGS compatible user credentials
   # credentials = "/etc/telegraf/nats.creds"
 
+  ## Optional authentication with nkey seed file (NATS 2.0)
+  # nkey_seed = "/etc/telegraf/seed.txt"
+
   ## NATS subject for producer messages.
   ## This field can be a static string or a Go template, see README for details.
   ## Incompatible with `use_batch_format