task(ui): Add TimelineMetadata::subscriber_skip_count.

This patch adds the `subscriber_skip_count` field to `TimelineMetadata`.
It's going to be used to define the `count` value of the `Skip`
higher-order stream that is going to be applied to the `Timeline`
subscriber.

crates/matrix-sdk-ui/src/timeline/controller/metadata.rs

@@ -20,7 +20,8 @@ use tracing::trace;
 
 use super::{
     super::{
-        reactions::Reactions, rfind_event_by_id, TimelineItem, TimelineItemKind, TimelineUniqueId,
+        reactions::Reactions, rfind_event_by_id, subscriber::skip::SkipCount, TimelineItem,
+        TimelineItemKind, TimelineUniqueId,
     },
     read_receipts::ReadReceipts,
     state::PendingPollEvents,
@@ -37,6 +38,8 @@ pub(in crate::timeline) struct TimelineMetadata {
     /// This value is constant over the lifetime of the metadata.
     internal_id_prefix: Option<String>,
 
+    pub(super) subscriber_skip_count: SkipCount,
+
     /// The hook to call whenever we run into a unable-to-decrypt event.
     ///
     /// This value is constant over the lifetime of the metadata.
@@ -107,6 +110,7 @@ impl TimelineMetadata {
         is_room_encrypted: Option<bool>,
     ) -> Self {
         Self {
+            subscriber_skip_count: SkipCount::new(),
             own_user_id,
             next_internal_id: Default::default(),
             reactions: Default::default(),

crates/matrix-sdk-ui/src/timeline/subscriber.rs

@@ -59,7 +59,7 @@ pub mod skip {
 
     const MAXIMUM_NUMBER_OF_INITIAL_ITEMS: usize = 20;
 
-    #[derive(Clone)]
+    #[derive(Clone, Debug)]
     pub struct SkipCount {
         count: SharedObservable<usize>,
     }