Re-initialize plugins only when needed (#19158)

Previously, for every config change, we would re-initialize
plugins irrespective of whether there would be any actual
plugin related changed or not.

Now, we compute the difference between previous and new configs
and check whether there has been any changes to plugin related
configuration, and re-init plugins only if that's true.

This improves performance as plugin re-initialization is quite
costly.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/bed59298-27da-4b74-8be5-cdeb4c50c3e2
```release-note
NONE
```

app/channels.go

@@ -5,15 +5,18 @@ package app
 
 import (
 	"runtime"
+	"strings"
 	"sync"
 	"sync/atomic"
 
 	"github.com/mattermost/mattermost-server/v6/app/imaging"
 	"github.com/mattermost/mattermost-server/v6/app/request"
+	"github.com/mattermost/mattermost-server/v6/config"
 	"github.com/mattermost/mattermost-server/v6/einterfaces"
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/plugin"
 	"github.com/mattermost/mattermost-server/v6/services/imageproxy"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 	"github.com/pkg/errors"
 )
 
@@ -119,11 +122,32 @@ func (ch *Channels) Start() error {
 	ch.initPlugins(ctx, *ch.srv.Config().PluginSettings.Directory, *ch.srv.Config().PluginSettings.ClientDirectory)
 
 	ch.AddConfigListener(func(prevCfg, cfg *model.Config) {
-		if *cfg.PluginSettings.Enable {
-			ch.initPlugins(ctx, *cfg.PluginSettings.Directory, *ch.srv.Config().PluginSettings.ClientDirectory)
-		} else {
-			ch.ShutDownPlugins()
+		// We compute the difference between configs
+		// to ensure we don't re-init plugins unnecessarily.
+		diffs, err := config.Diff(prevCfg, cfg)
+		if err != nil {
+			mlog.Warn("Error in comparing configs", mlog.Err(err))
+			return
 		}
+
+		hasDiff := false
+		// TODO: This could be a method on ConfigDiffs itself
+		for _, diff := range diffs {
+			if strings.HasPrefix(diff.Path, "PluginSettings.") {
+				hasDiff = true
+				break
+			}
+		}
+
+		// Do only if some plugin related settings has changed.
+		if hasDiff {
+			if *cfg.PluginSettings.Enable {
+				ch.initPlugins(ctx, *cfg.PluginSettings.Directory, *ch.srv.Config().PluginSettings.ClientDirectory)
+			} else {
+				ch.ShutDownPlugins()
+			}
+		}
+
 	})
 
 	if err := ch.ensureAsymmetricSigningKey(); err != nil {