fix unclosed Thread when create TiSession error (#300)

[ti-srebot]

cherry-pick #300 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/301

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

git push [email protected]:ti-srebot/client-java.git pr/301:release-3.1-489e5c22ec7c

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Signed-off-by: marsishandsome [email protected]

What problem does this PR solve?

If the metrics HTTP port is not free, TiSession will leave a unclosed Thread in PDClient.

What is changed and how it works?

put MetricsServer.getInstance at the beginning of this function to avoid unclosed Thread

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

[ti-srebot]

/run-all-tests

[marsishandsome]

LGTM