MM-49984: Displaying active search backend in system console (#22721)

This will be shown in the system console to let users know
which search backend is active.

Implemented via adding an extra param in the ping response.

https://mattermost.atlassian.net/browse/MM-49984

```release-note
The database section in the system console now has an additional
read-only section which shows the active search backend in use.

This can be helpful to confirm which is the currently active
search engine when there are multiple of them configured.
```
---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/app_iface.go

@@ -400,6 +400,7 @@ type AppIface interface {
 	VerifyPlugin(plugin, signature io.ReadSeeker) *model.AppError
 	AccountMigration() einterfaces.AccountMigrationInterface
 	ActivateMfa(userID, token string) *model.AppError
+	ActiveSearchBackend() string
 	AddChannelsToRetentionPolicy(policyID string, channelIDs []string) *model.AppError
 	AddConfigListener(listener func(*model.Config, *model.Config)) string
 	AddDirectChannels(c request.CTX, teamID string, user *model.User) *model.AppError

server/channels/app/opentracing/opentracing_layer.go

@@ -89,6 +89,23 @@ func (a *OpenTracingAppLayer) ActivateMfa(userID string, token string) *model.Ap
 	return resultVar0
 }
 
+func (a *OpenTracingAppLayer) ActiveSearchBackend() string {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.ActiveSearchBackend")
+
+	a.ctx = newCtx
+	a.app.Srv().Store().SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store().SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0 := a.app.ActiveSearchBackend()
+
+	return resultVar0
+}
+
 func (a *OpenTracingAppLayer) AddChannelMember(c request.CTX, userID string, channel *model.Channel, opts app.ChannelMemberOpts) (*model.ChannelMember, *model.AppError) {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.AddChannelMember")

server/channels/app/searchengine.go

@@ -60,3 +60,7 @@ func (a *App) PurgeBleveIndexes() *model.AppError {
 	}
 	return nil
 }
+
+func (a *App) ActiveSearchBackend() string {
+	return a.ch.srv.platform.SearchEngine.ActiveEngine()
+}