Support metrics with multiple TiSessions with the same port (#220)

[marsishandsome]

cherry-pick #220 to release-3.1
You can switch your code base to this Pull Request by using git-extras:

# In client-java repo:
git pr https://github.com/tikv/client-java/pull/221

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/client-java.git pr/221:release-3.1-630e1513e9cf

---

Signed-off-by: marsishandsome marsishandsome@gmail.com

What problem does this PR solve?

close #218

Currently, only one TiSession is supported with metrics exporting.

What is changed and how it works?

• Support metrics with multiple TiSessions with the same port
• Do not support multiple TiSessions with different ports

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[marsishandsome]

/run-all-tests

[birdstorm]

LGTM