feat(output.zabbix): use system hostname if no host tag is found

If the host tag is not present in one metric, instead of failing, try to
use the system hostname.

plugins/outputs/zabbix/lld.go

@@ -3,8 +3,9 @@ package zabbix
 import (
 	"encoding/binary"
 	"encoding/json"
-	"errors"
+	"fmt"
 	"hash/fnv"
+	"os"
 	"sort"
 	"strings"
 	"time"
@@ -198,8 +199,13 @@ func (zl *zabbixLLD) Add(in telegraf.Metric) error {
 		key = in.Name() + "." + strings.Join(keys, ".")
 	}
 
+	// If hostname is not defined, use the hostname of the system
 	if hostname == "" {
-		return errors.New("no hostname found")
+		var err error
+		hostname, err = os.Hostname()
+		if err != nil {
+			return fmt.Errorf("no hostname found and unable to get hostname from system: %w", err)
+		}
 	}
 
 	// Try to lookup the Zabbix series in the already received metrics and