*: add metrics-min-interval to improve the chance of collecting top-heavy metrics

[zimulala]

What problem does this PR solve?

This is a small part extracted from PR #123.

What is changed and how it works?

• If pulling top-heavy metrics fails, you can consider reducing the time range for each request. The metrics-low-priority flag is used to reduce the time range of each request.

  • However, this may result in much slower data retrieval.

• Optimized connection pool management (use default http.Client)

  • By customizing the connection pool size and layered timeouts, blocked connections are prevented from affecting requests, improving concurrency stability.

Check List

Tests

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

    tiup diag collect debug \
  --exclude "log,system,db_vars" \
  --include "monitor" \
  --output "/root/prom-2k" \
  --from "2025-11-25T16:50:14+08:00" \
  --to "2025-11-25T17:10:14+08:00" \
  -R "tidb"

Before this PR

In this case, one file every 5 minutes, and the following three tests only get 5 minutes of data for each

After this PR

with --metrics-min-interval "300"
In this case, one file every 5 minutes, and the following three tests get 10 minutes of data for each

with default metrics-min-interval(120)
In this case, one file every 2 minutes, and we have 3 runs, the first with 12 minutes of data (6 files), and the next two tests with 20 minutes of data (10 files)

• 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

[yibin87]

LGTM

[kaaaaaaang]

rest LGT @mornyx

[kaaaaaaang]

I think it is not used anymore

[zimulala]

This is the minimum protection value. Because if the value is lower than this, the proportion of metadata will be high. If the segmentation is too small, it would be meaningless.

[kaaaaaaang]

is 120s the new default value?

[zimulala]

Yes, if you think it's best to keep it the same as before, I can change it to 5 minutes.

[zimulala]

PTAL @mornyx