syncer: add metric replicationLagGauge back (#1709)

[ti-chi-bot]

This is an automated cherry-pick of #1709

What problem does this PR solve?

in #1451

• Part 1 fix replicationLagGauge

What is changed and how it works?

use DM current ts minus binlog header ts to get the replication lag

Check List

Tests

• Unit test
• Integration test
• Manual test
  1. start any incremental task in any dm-worker
  2. check the workers metrics , you can find the dm_syncer_replication_lag back again

Code changes

• Has exported function/method change
• Add GetServerUnixTS

Side effects

• Possible performance regression

1. need get the time offset between DM and upstream db in background task Regularly

Related changes

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

[ti-chi-bot]

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[lichunzhu]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lance6716
• lichunzhu

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[Ehco1996]

/run-unit-test

[lance6716]

/merge

[ti-chi-bot]

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

Commit hash: 401d12d