Message export: Fix job progress calculation (#21925)

* use posts count instead of job duration to calculate progress

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/post.go

@@ -333,13 +333,16 @@ type GetPostsOptions struct {
 
 type PostCountOptions struct {
 	// Only include posts on a specific team. "" for any team.
-	TeamId          string
-	MustHaveFile    bool
-	MustHaveHashtag bool
-	ExcludeDeleted  bool
-	UsersPostsOnly  bool
+	TeamId             string
+	MustHaveFile       bool
+	MustHaveHashtag    bool
+	ExcludeDeleted     bool
+	ExcludeSystemPosts bool
+	UsersPostsOnly     bool
 	// AllowFromCache looks up cache only when ExcludeDeleted and UsersPostsOnly are true and rest are falsy.
 	AllowFromCache bool
+	SincePostID    string
+	SinceUpdateAt  int64
 }
 
 func (o *Post) Etag() string {