Refine debug logs of querying extensions (#6414)

#### What type of PR is this?

/kind cleanup
/area core
/milestone 2.18.x

#### What this PR does / why we need it:

This PR refines debug logs of listing all extensions.

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/main/java/run/halo/app/extension/ReactiveExtensionClientImpl.java

@@ -140,10 +140,10 @@ public <E extends Extension> Flux<E> listAll(Class<E> type, ListOptions options,
                 final long startTimeMs = System.currentTimeMillis();
                 return client.listByNames(storeNames)
                     .map(extensionStore -> converter.convertFrom(type, extensionStore))
-                    .doOnNext(s -> {
-                        log.debug("Successfully retrieved all by names from db for {} in {}ms",
-                            scheme.groupVersionKind(), System.currentTimeMillis() - startTimeMs);
-                    });
+                    .doOnComplete(() -> log.debug(
+                        "Successfully retrieved all by names from db for {} in {}ms",
+                        scheme.groupVersionKind(), System.currentTimeMillis() - startTimeMs)
+                    );
             });
     }
 
@@ -161,10 +161,10 @@ public <E extends Extension> Mono<ListResult<E>> listBy(Class<E> type, ListOptio
                 final long startTimeMs = System.currentTimeMillis();
                 return client.listByNames(storeNames)
                     .map(extensionStore -> converter.convertFrom(type, extensionStore))
-                    .doOnNext(s -> {
-                        log.debug("Successfully retrieved by names from db for {} in {}ms",
-                            scheme.groupVersionKind(), System.currentTimeMillis() - startTimeMs);
-                    })
+                    .doOnComplete(() -> log.debug(
+                        "Successfully retrieved by names from db for {} in {}ms",
+                        scheme.groupVersionKind(), System.currentTimeMillis() - startTimeMs)
+                    )
                     .collectList()
                     .map(result -> new ListResult<>(page.getPageNumber(), page.getPageSize(),
                         objectKeys.getTotal(), result));