feat(agent): Enable watching for new files

This allows for watching for new files in a configuraiton directory.
When a new file is found, Telegraf will reload with that file added to
the list of files to watch. Note this will reload when any file, a
telegraf config or not, is added to the directory.

TODO: notify type poll does not seem to work, only inotify
TODO: test on Windows

fixes: #12389
fixes: #12264

cmd/telegraf/telegraf.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"strings"
 	"syscall"
 	"time"
@@ -157,6 +158,18 @@ func (t *Telegraf) reloadLoop() error {
 					go t.watchLocalConfig(ctx, signals, fConfig)
 				}
 			}
+			for _, fConfigDirectory := range t.configDir {
+				absPath, err := filepath.Abs(fConfigDirectory)
+				if err != nil {
+					log.Printf("W! Cannot watch config directory %s: %s", fConfigDirectory, err)
+					continue
+				}
+				if _, err := os.Stat(absPath); err != nil {
+					log.Printf("W! Cannot watch config directory %s: %s", fConfigDirectory, err)
+				} else {
+					go t.watchLocalDirectory(ctx, signals, absPath)
+				}
+			}
 		}
 		if t.configURLWatchInterval > 0 {
 			remoteConfigs := make([]string, 0)
@@ -240,6 +253,32 @@ func (t *Telegraf) watchLocalConfig(ctx context.Context, signals chan os.Signal,
 	signals <- syscall.SIGHUP
 }
 
+func (t *Telegraf) watchLocalDirectory(ctx context.Context, signals chan os.Signal, directory string) {
+	var mytomb tomb.Tomb
+	var watcher watch.FileWatcher
+	if t.watchConfig == "poll" {
+		watcher = watch.NewPollingFileWatcher(directory)
+	} else {
+		watcher = watch.NewInotifyFileWatcher(directory)
+	}
+	changes, err := watcher.ChangeEvents(&mytomb, 0)
+	if err != nil {
+		log.Printf("E! Error watching config directory %q: %s\n", directory, err)
+		return
+	}
+	log.Printf("I! Config watcher started for directory %s\n", directory)
+	select {
+	case <-ctx.Done():
+		mytomb.Done()
+	case <-changes.Created:
+		log.Printf("I! Config directory %q has new file(s)\n", directory)
+		mytomb.Done()
+		signals <- syscall.SIGHUP
+	case <-mytomb.Dying():
+		log.Printf("I! Config directory watcher %q ended\n", directory)
+	}
+}
+
 func (t *Telegraf) watchRemoteConfigs(ctx context.Context, signals chan os.Signal, interval time.Duration, remoteConfigs []string) {
 	configs := strings.Join(remoteConfigs, ", ")
 	log.Printf("I! Remote config watcher started for: %s\n", configs)