[MM-44263] Cache Posts Usage (#20254)

* Fetch users only posts

* Cache Posts Usage

* Explicitly validate posts count before testing usage

* Add DocString

* Revert cache changes

* Revert "Revert cache changes"

This reverts commit ba693f8d5028b62c19f6a730e8dca73a224d0761.

app/usage.go

@@ -47,7 +47,7 @@ func (ch *Channels) getIntegrationsUsage() (*model.IntegrationsUsage, *model.App
 
 // GetPostsUsage returns "rounded off" total posts count like returns 900 instead of 987
 func (a *App) GetPostsUsage() (int64, *model.AppError) {
-	count, err := a.Srv().Store.Post().AnalyticsPostCount(&model.PostCountOptions{ExcludeDeleted: true, UsersPostsOnly: true})
+	count, err := a.Srv().Store.Post().AnalyticsPostCount(&model.PostCountOptions{ExcludeDeleted: true, UsersPostsOnly: true, AllowFromCache: true})
 	if err != nil {
 		return 0, model.NewAppError("GetPostsUsage", "app.post.analytics_posts_count.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}