client: update the leader even if the connection creation fails

[CabinfeverB]

What problem does this PR solve?

Issue Number: Close #7416

What is changed and how does it work?

update the leader even if the connection creation fails.
If connection is nil, consider it as networkErr

Check List

Tests

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

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

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

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• rleungx

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[codecov]

Codecov Report

Merging #7443 (dd5627d) into master (1934450) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7443      +/-   ##
==========================================
- Coverage   74.31%   74.30%   -0.02%     
==========================================
  Files         451      451              
  Lines       49735    49745      +10     
==========================================
+ Hits        36960    36962       +2     
+ Misses       9453     9448       -5     
- Partials     3322     3335      +13     

Flag	Coverage Δ
unittests	74.30% <100.00%> (-0.02%)	⬇️

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[CabinfeverB]

/merge

[ti-chi-bot]

@CabinfeverB: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: d926df2

[ti-chi-bot]

@CabinfeverB: Your PR was out of date, I have automatically updated it for you.

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #7473.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #7474.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #7475.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #7476.