trace memory of MemTables

[tabokie]

Signed-off-by: tabokie xy.tao@outlook.com

Ref #205

Added a new method to flush internal metrics, including memory used by MemTables.

[codecov]

Codecov Report

Merging #207 (303b9c5) into master (bac5ec4) will increase coverage by 0.01%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   96.95%   96.96%   +0.01%     
==========================================
  Files          28       28              
  Lines        7652     7687      +35     
==========================================
+ Hits         7419     7454      +35     
  Misses        233      233              

Impacted Files	Coverage Δ
src/engine.rs	97.13% <95.00%> (-0.04%)	⬇️
src/memtable.rs	99.13% <100.00%> (+<0.01%)	⬆️
src/metrics.rs	97.58% <100.00%> (+0.08%)	⬆️
src/consistency.rs	93.33% <0.00%> (+2.22%)	⬆️

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[tabokie]

I ran a benchmark to get a baseline memory usage:

# ./target/release/stress --path /data3/re_bench --time 60 --regions 1000 --write-threads 1 --purge-interval 0 --compact-count 0 --write-without-sync
Memory usage: 8003MiB
Throughput(QPS) = 30061.84
Latency(μs) min = 8, avg = 17.72, p50 = 15, p90 = 28, p95 = 33, p99 = 48, p99.9 = 76, max = 5827
Fairness = 100.0%
Write Bandwidth = 22.5MiB/s

[Connor1996]

LGTM