expression: fix casting REAL type to STRING type

[solotzg]

What is changed and how it works?

Issue Number: Close #16974

ref pingcap/tidb#53129

What's Changed:

expression: fix casting REAL type to STRING type
- unify the behavior of converting real types to strings in tidb#53129 and tikv.
- use ryu lib in tikv to implement casting real type to str type.
- `if abs(num) >= 1e15 or (num is not zero and abs(num) < 1e-15)` then use decimal exponent format 

Related changes

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

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Release note

Fix the incorrect result when casting REAL type to STRING type

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• gengliqi
• wshwsh12

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-chi-bot]

Hi @solotzg. Thanks for your PR.

I'm waiting for a tikv member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

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 kubernetes/test-infra repository.

[solotzg]

/cc @wshwsh12 @gengliqi

[solotzg]

/ok-to-test

[ti-chi-bot]

@solotzg: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/ok-to-test

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 kubernetes/test-infra repository.

[gengliqi]

/ok-to-test

[gengliqi]

/merge

[ti-chi-bot]

@gengliqi: 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.

If you have any questions about the PR merge process, please refer to pr process.

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]

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

Commit hash: ef1f2e5

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.4: #16998.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #16999.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #17000.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #17001.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #17002.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17003.