[MM-43917] Cloud Freemium limits API: messages/posts (#20152)

* WIP - Add api and app funcs

* Add test cases

* Add utils testcases

* Exclude deleted posts

* Add doc for func

* Move api from cloud to usage

* Allow api access to authenticated users

* Change int to int64

* Fix lint issue

* Simplify err check

Co-authored-by: Ashish Bhate <ashish.bhate@mattermost.com>

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Ashish Bhate <ashish.bhate@mattermost.com>

app/usage.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/utils"
+)
+
+// 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})
+	if err != nil {
+		return 0, model.NewAppError("GetPostsUsage", "app.post.analytics_posts_count.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+
+	return utils.RoundOffToZeroes(float64(count)), nil
+}