br: backup: Add backup result into backupMeta

[joccau]

What problem does this PR solve?

Issue Number: close issue(pingcap/br#1420)

Problem Summary: Add the backup result into backupMeta

What is changed and how it works?

What's Changed:
Add a member(backup_result) in brpb.proto. Write the backupResult when Flush the backupmeta into storage

Note

The changes of brpb in pr(pingcap/kvproto#793)
After the pr(pingcap/kvproto#793) is merged, it can add merge flag in this PR

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Manual Test

./br backup full --pd "172.16.6.117:2379" --storage "local:///home/zak/backup_del" --check-requirements=false

The log

The tail part in backupmeta in storage

Release note

Add the backup result into `backupMeta` in the storage

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[joccau]

/cc @3pointer
/cc @YuJuncen
/cc @glorv
/cc @kennytm
/cc @IANTHEREAL

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@joccau: GitHub didn't allow me to request PR reviews from the following users: YuJuncen.

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In response to this:

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[joccau]

/sig migrate

[joccau]

/component br

[YuJuncen]

It there some reason we must use the log-like format here? Considering that our users usually won't read the backup meta directly, the format could be more friendly to computers. I would prefer to write them as a more structural format, say, JSON?
(Another benefit of using json is that we can use the zap itself to encode it)

[ti-chi-bot]

@joccau: PR needs rebase.

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@joccau: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
tiprow_fast_test	4115309	link	true	/test fast_test_tiprow

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[ti-chi-bot]

@joccau: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	4115309	link	true	/test build
idc-jenkins-ci-tidb/mysql-test	4115309	link	true	/test mysql-test
pull-integration-ddl-test	4115309	link	true	/test pull-integration-ddl-test
idc-jenkins-ci-tidb/unit-test	4115309	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	4115309	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	4115309	link	true	/test check-dev2
pull-mysql-client-test	4115309	link	true	/test pull-mysql-client-test
pull-lightning-integration-test	4115309	link	true	/test pull-lightning-integration-test
pull-br-integration-test	4115309	link	true	/test pull-br-integration-test

Full PR test history. Your PR dashboard.

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