add example for SessionPool

[Nicole00]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

close #493

Description:

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

[codecov-commenter]

Codecov Report

Base: 63.46% // Head: 63.41% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (81ca163) compared to base (a719bd5).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #500      +/-   ##
============================================
- Coverage     63.46%   63.41%   -0.06%     
+ Complexity      787      786       -1     
============================================
  Files            71       71              
  Lines          3758     3758              
  Branches        537      537              
============================================
- Hits           2385     2383       -2     
- Misses         1004     1005       +1     
- Partials        369      370       +1     

Impacted Files	Coverage Δ
...va/com/vesoft/nebula/client/graph/SessionPool.java	56.49% <0.00%> (-1.13%)	⬇️

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