ddl: add owner tenure metric and panel

[fzzf678]

What problem does this PR solve?

Issue Number: ref #65587

Problem Summary:

• Oncall lacks a clear way to see which TiDB instance is the DDL owner and how long it has held ownership; owner switch visibility is limited.

What changed and how does it work?

• Add gauge metric tidb_owner_is_owner_since_seconds{type="ddl"} (and other owner types via type) set to the Unix epoch seconds when an instance becomes owner, and reset to 0 when it is not owner.
• Add a Grafana panel DDL Owner to visualize owner tenure (time() - tidb_owner_is_owner_since_seconds) and owner switches.

Check List

Tests

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

  • I checked and no code files have been changed.

Manual test steps:

1. Deploy a TiDB cluster with multiple TiDB instances.
2. Verify tidb_owner_is_owner_since_seconds{type="ddl"} is > 0 only on the DDL owner instance and 0 on others.
3. Trigger an owner switch (restart the owner TiDB) and verify the metric resets and starts increasing from ~0 on the new owner.
4. Import updated dashboards and verify DDL Owner panel shows the owner tenure and switch events.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Added `tidb_owner_is_owner_since_seconds` metric and a Grafana panel to visualize DDL owner tenure and owner switches.

[ti-chi-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.6082%. Comparing base (53dc671) to head (80a7216).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65590        +/-   ##
================================================
- Coverage   77.8520%   77.6082%   -0.2439%     
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  Files          1973       1902        -71     
  Lines        541968     530181     -11787     
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- Hits         421933     411464     -10469     
- Misses       118376     118709       +333     
+ Partials       1659          8      -1651     

Flag	Coverage Δ
integration	41.7716% <0.0000%> (-6.4182%)	⬇️
unit	76.7700% <100.0000%> (+0.2928%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	56.7974% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8219% <ø> (-12.3231%)	⬇️

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