support dumping metrics by instance

[zyguan]

What problem does this PR solve?

For a large cluster, getSeriesNum may always fail due to quota limit. As a result, significant metrics like tidb_tikvclient_request_seconds and tikv_grpc_msg_duration_seconds are often missing, which blocks troubleshooting.

What is changed and how it works?

With this PR, if getSeriesNum failed, diag will fallback to by-instance dump mode, which get instance list first and call collectMetric by instance.

Check List

Tests

• [ ] Unit test
• [ ] Integration test
• [x] 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

• [x] Possible performance regression
• [ ] Increased code complexity
• [ ] Breaking backward compatibility

Related changes

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

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4c0b393) 30.21% compared to head (913f819) 30.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #470 +/- ## ======================================= Coverage 30.21% 30.21% ======================================= Files 52 52 Lines 4416 4416 ======================================= Hits 1334 1334 Misses 2911 2911 Partials 171 171 ``` | [Flag](https://app.codecov.io/gh/pingcap/diag/pull/470/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/470/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `30.21% <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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