Add interface to get the raft-engine path.

[jiayang-zheng]

Related PR: tikv/tikv#13633

Add an interface to expose the path of raft engine to calculate raft engine capacity.

Signed-off-by: Jarvis Zheng jiayang@hust.edu.cn

[codecov]

Codecov Report

Base: 97.65% // Head: 97.65% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f3ccc22) compared to base (a0d2998).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          30       30           
  Lines       10630    10634    +4     
=======================================
+ Hits        10381    10385    +4     
  Misses        249      249           

Impacted Files	Coverage Δ
src/engine.rs	97.90% <100.00%> (+<0.01%)	⬆️
src/pipe_log.rs	93.93% <0.00%> (-1.52%)	⬇️
src/env/default.rs	92.81% <0.00%> (+0.55%)	⬆️

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