only collect metrics from the first prometheus

[nexustar]

What problem does this PR solve?

• close #461

What is changed and how it works?

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

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

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (28c2e01) 30.19% compared to head (fee1243) 30.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #462 +/- ## ======================================= Coverage 30.19% 30.19% ======================================= Files 52 52 Lines 4415 4415 ======================================= Hits 1333 1333 Misses 2911 2911 Partials 171 171 ``` | [Flag](https://app.codecov.io/gh/pingcap/diag/pull/462/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pingcap/diag/pull/462/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `30.19% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap#carryforward-flags-in-the-pull-request-comment) to find out more.

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