PD store usage not right in master.

[jiaqizho]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

• The tiflash used from tiflash metrics

• The metrics from pd

They both called getFsStats(). And but get the different result.
I think the problem in pd.rs handle_store_heartbeat:

let capacity = store_stats.fs_stats.capacity_size;
        let available = store_stats.fs_stats.avail_size;
        stats.set_used_size(store_stats.fs_stats.used_size);
        stats.set_capacity(capacity);

        let used_size = self.snap_mgr.get_total_snap_size().unwrap()
            + store_info
                .kv_engine
                .get_engine_used_size()
                .expect("kv engine used size")
            + store_info
                .raft_engine
                .get_engine_size()
                .expect("raft engine used size");
        stats.set_used_size(used_size);

The below set_used_size replace the origin one.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)