*: refine metrics (#1438)

[ti-srebot]

cherry-pick #1438 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1443

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

git push git@github.com:ti-srebot/dm.git pr/1443:release-2.0-2fa864f33260

---

What problem does this PR solve?

part of #1218
part of #1419

What is changed and how it works?

• don't overwrite prometheus registry so metrics of embedded etcd could be exposed
• add alert when no master is up
• fix prometheus query error of one-to-many subtraction
• remove duplicate copys of metrics config files

Check List

Tests

• Integration test
• Manual test (check group_right could work)

Code changes

Side effects

Related changes

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