[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>

api4/usage.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package api4
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/v6/model"
+)
+
+func (api *API) InitUsage() {
+	// GET /api/v4/usage/posts
+	api.BaseRoutes.Usage.Handle("/posts", api.APISessionRequired(getPostsUsage)).Methods("GET")
+}
+
+func getPostsUsage(c *Context, w http.ResponseWriter, r *http.Request) {
+	count, appErr := c.App.GetPostsUsage()
+	if appErr != nil {
+		c.Err = model.NewAppError("Api4.getPostsUsage", "app.post.analytics_posts_count.app_error", nil, appErr.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	json, err := json.Marshal(&model.PostsUsage{Count: count})
+	if err != nil {
+		c.Err = model.NewAppError("Api4.getPostsUsage", "api.marshal_error", nil, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	w.Write(json)
+}