fix: page stats does not subscribe in finder (#3003)

#### What type of PR is this?
/kind bug
/area core
#### What this PR does / why we need it:
修复自定义页面 Finder 中 getByName 方法没有返回访问量的问题
此问题为重构 #2918 时引入。

#### Which issue(s) this PR fixes:

Fixes #2992

#### Special notes for your reviewer:
/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?
```release-note
None
```

src/main/java/run/halo/app/theme/finders/impl/SinglePageFinderImpl.java

@@ -61,9 +61,12 @@ public class SinglePageFinderImpl implements SinglePageFinder {
                 SinglePageVo pageVo = SinglePageVo.from(page);
                 pageVo.setContributors(List.of());
                 pageVo.setContent(ContentVo.empty());
-                populateStats(pageVo);
                 return pageVo;
             })
+            .flatMap(singlePageVo -> fetchStats(singlePageVo)
+                .doOnNext(singlePageVo::setStats)
+                .thenReturn(singlePageVo)
+            )
             .flatMap(this::populateContributors)
             .flatMap(page -> content(pageName)
                 .doOnNext(page::setContent)
@@ -94,7 +97,7 @@ public class SinglePageFinderImpl implements SinglePageFinder {
                     pageVo.setContributors(List.of());
                     return pageVo;
                 })
-                .flatMap(lp -> populateStats(lp).doOnNext(lp::setStats).thenReturn(lp))
+                .flatMap(lp -> fetchStats(lp).doOnNext(lp::setStats).thenReturn(lp))
                 .concatMap(this::populateContributors)
                 .collectList()
                 .map(pageVos -> new ListResult<>(list.getPage(), list.getSize(), list.getTotal(),
@@ -104,7 +107,7 @@ public class SinglePageFinderImpl implements SinglePageFinder {
             .defaultIfEmpty(new ListResult<>(0, 0, 0, List.of()));
     }
 
-    <T extends ListedSinglePageVo> Mono<StatsVo> populateStats(T pageVo) {
+    <T extends ListedSinglePageVo> Mono<StatsVo> fetchStats(T pageVo) {
         String name = pageVo.getMetadata().getName();
         return counterService.getByName(MeterUtils.nameOf(SinglePage.class, name))
             .map(counter -> StatsVo.builder()

src/test/java/run/halo/app/theme/finders/impl/SinglePageFinderImplTest.java

@@ -0,0 +1,79 @@
+package run.halo.app.theme.finders.impl;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+import run.halo.app.content.ContentService;
+import run.halo.app.core.extension.content.SinglePage;
+import run.halo.app.extension.Metadata;
+import run.halo.app.extension.ReactiveExtensionClient;
+import run.halo.app.metrics.CounterService;
+import run.halo.app.theme.finders.ContributorFinder;
+
+/**
+ * Tests for {@link SinglePageFinderImpl}.
+ *
+ * @author guqing
+ * @since 2.0.1
+ */
+@ExtendWith(MockitoExtension.class)
+class SinglePageFinderImplTest {
+
+    @Mock
+    private ReactiveExtensionClient client;
+
+    @Mock
+    private ContentService contentService;
+
+    @Mock
+    private ContributorFinder contributorFinder;
+
+    @Mock
+    private CounterService counterService;
+
+    @InjectMocks
+    private SinglePageFinderImpl singlePageFinder;
+
+    @Test
+    void getByName() {
+        // fix gh-2992
+        String fakePageName = "fake-page";
+        SinglePage singlePage = new SinglePage();
+        singlePage.setMetadata(new Metadata());
+        singlePage.getMetadata().setName(fakePageName);
+        singlePage.setSpec(new SinglePage.SinglePageSpec());
+        singlePage.getSpec().setOwner("fake-owner");
+        singlePage.getSpec().setReleaseSnapshot("fake-release");
+        singlePage.setStatus(new SinglePage.SinglePageStatus());
+        when(client.fetch(eq(SinglePage.class), eq(fakePageName)))
+            .thenReturn(Mono.just(singlePage));
+
+        when(counterService.getByName(anyString())).thenReturn(Mono.empty());
+        when(contributorFinder.getContributor(anyString())).thenReturn(Mono.empty());
+        when(contentService.getContent(anyString())).thenReturn(Mono.empty());
+
+        singlePageFinder.getByName(fakePageName)
+            .as(StepVerifier::create)
+            .consumeNextWith(page -> {
+                assertThat(page.getStats()).isNotNull();
+                assertThat(page.getContent()).isNotNull();
+            })
+            .verifyComplete();
+
+        verify(client, times(2)).fetch(SinglePage.class, fakePageName);
+        verify(counterService).getByName(anyString());
+        verify(contentService).getContent(anyString());
+        verify(contributorFinder).getContributor(anyString());
+    }
+}