fix #1380, create NacosRegistryProviderObserver when init method is executed

[wangchengming666]

fix #1380 修改之前的代码创建NacosRegistryProviderObserver是在subscribe里的，修改之后创建NacosRegistryProviderObserver是在init方法里的，init是有同步锁的。

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.03%. Comparing base (df2dcae) to head (2598309).

:exclamation: Current head 2598309 differs from pull request most recent head d68be0a. Consider uploading reports for the commit d68be0a to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1392 +/- ## ========================================= Coverage 72.02% 72.03% - Complexity 791 797 +6 ========================================= Files 422 422 Lines 17813 17813 Branches 2769 2769 ========================================= + Hits 12830 12831 +1 - Misses 3568 3570 +2 + Partials 1415 1412 -3 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.