MM-53669: Use the cache layer for EmojiStore.GetMultipleByName (#24030)

There was already a cache present for emoji names. But we weren't using
it for the GetMultipleByName method. Now we implement that method
to look up the cache for every emoji name passed.

Secondly, a bigger problem was that we were making the DB call for system
emojis as well. Since system emojis aren't stored in the DB, it would
fall through the cache layer and always make a redundant DB call. In the
profiles, this should up as taking 16% of the total time to serve
a getPostsForChannel API endpoint.

We fix this by filtering the emojis to only custom emojis
before making the call.

https://mattermost.atlassian.net/browse/MM-53669

```release-note
NONE
```

server/channels/store/localcachelayer/emoji_layer_test.go

@@ -21,6 +21,7 @@ func TestEmojiStore(t *testing.T) {
 
 func TestEmojiStoreCache(t *testing.T) {
 	fakeEmoji := model.Emoji{Id: "123", Name: "name123"}
+	fakeEmoji2 := model.Emoji{Id: "321", Name: "name321"}
 	ctxEmoji := model.Emoji{Id: "master", Name: "name123"}
 
 	t.Run("first call by id not cached, second cached and returning same data", func(t *testing.T) {
@@ -39,7 +40,7 @@ func TestEmojiStoreCache(t *testing.T) {
 		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "Get", 1)
 	})
 
-	t.Run("first call by name not cached, second cached and returning same data", func(t *testing.T) {
+	t.Run("GetByName: first call by name not cached, second cached and returning same data", func(t *testing.T) {
 		mockStore := getMockStore()
 		mockCacheProvider := getMockCacheProvider()
 		cachedStore, err := NewLocalCacheLayer(mockStore, nil, nil, mockCacheProvider)
@@ -55,6 +56,43 @@ func TestEmojiStoreCache(t *testing.T) {
 		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetByName", 1)
 	})
 
+	t.Run("GetMultipleByName: first call by name not cached, second cached and returning same data", func(t *testing.T) {
+		mockStore := getMockStore()
+		mockCacheProvider := getMockCacheProvider()
+		cachedStore, err := NewLocalCacheLayer(mockStore, nil, nil, mockCacheProvider)
+		require.NoError(t, err)
+
+		emojis, err := cachedStore.Emoji().GetMultipleByName(context.Background(), []string{"name123"})
+		require.NoError(t, err)
+		require.Len(t, emojis, 1)
+		assert.Equal(t, emojis[0], &fakeEmoji)
+		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetMultipleByName", 1)
+		emojis, err = cachedStore.Emoji().GetMultipleByName(context.Background(), []string{"name123"})
+		require.NoError(t, err)
+		require.Len(t, emojis, 1)
+		assert.Equal(t, emojis[0], &fakeEmoji)
+		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetMultipleByName", 1)
+	})
+
+	t.Run("GetMultipleByName: multiple elements", func(t *testing.T) {
+		mockStore := getMockStore()
+		mockCacheProvider := getMockCacheProvider()
+		cachedStore, err := NewLocalCacheLayer(mockStore, nil, nil, mockCacheProvider)
+		require.NoError(t, err)
+
+		emojis, err := cachedStore.Emoji().GetMultipleByName(context.Background(), []string{"name123", "name321"})
+		require.NoError(t, err)
+		require.Len(t, emojis, 2)
+		assert.Equal(t, emojis[0], &fakeEmoji)
+		assert.Equal(t, emojis[1], &fakeEmoji2)
+		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetMultipleByName", 1)
+		emojis, err = cachedStore.Emoji().GetMultipleByName(context.Background(), []string{"name123"})
+		require.NoError(t, err)
+		require.Len(t, emojis, 1)
+		assert.Equal(t, emojis[0], &fakeEmoji)
+		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetMultipleByName", 1)
+	})
+
 	t.Run("first call by id not cached, second force not cached", func(t *testing.T) {
 		mockStore := getMockStore()
 		mockCacheProvider := getMockCacheProvider()
@@ -107,7 +145,7 @@ func TestEmojiStoreCache(t *testing.T) {
 		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetByName", 2)
 	})
 
-	t.Run("first call by id, second call by name cached", func(t *testing.T) {
+	t.Run("first call by id, second call by name and GetMultipleByName cached", func(t *testing.T) {
 		mockStore := getMockStore()
 		mockCacheProvider := getMockCacheProvider()
 		cachedStore, err := NewLocalCacheLayer(mockStore, nil, nil, mockCacheProvider)
@@ -117,6 +155,8 @@ func TestEmojiStoreCache(t *testing.T) {
 		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "Get", 1)
 		cachedStore.Emoji().GetByName(context.Background(), "name123", true)
 		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetByName", 0)
+		cachedStore.Emoji().GetMultipleByName(context.Background(), []string{"name123"})
+		mockStore.Emoji().(*mocks.EmojiStore).AssertNumberOfCalls(t, "GetMultipleByName", 0)
 	})
 
 	t.Run("first call by name, second call by id cached", func(t *testing.T) {