fix: avoid acquiring lock during reading stats

[WenyXu]

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What's changed and what's your intention?

This PR uses Arc<Atomicu64> to store the shared value instead of RwLock, to avoid acquiring lock during reading value.

The datanode will report the stats (like manifest usage) via heartbeat. Before sending the heartbeat to the metasrv, it collects all stats from the region server. While collecting disk_usage will invoke the manifest_usage method on the manifest_manager, this operation is required to acquire the read lock of the manifest_manager.

       // read manifest_usage
       let manifest_usage = self
            .manifest_ctx
            .manifest_manager
            .read()
            .await
            .manifest_usage();

      let mut manager = self.manifest_manager.write().await;
      // update manifest
      // ..

However, acquiring a read lock will be blocked by the updating manifest operations, and the wait time may be as long as tens of seconds. This makes the datanode fail to send heartbeats in time, then all region leases on the datanodes will expire.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[killme2008]

Good catch!

[fengjiachun]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 96.05263% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 85.04%. Comparing base (6e9a9dc) to head (ee1b388).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4070      +/-   ##
==========================================
- Coverage   85.34%   85.04%   -0.31%     
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  Files         987      987              
  Lines      171485   172109     +624     
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+ Hits       146359   146371      +12     
- Misses      25126    25738     +612