Private/issue 2677 decision statistics pr desc clean

[ljluestc]

What problem does this PR solve?

Issue Number: Close #1125

When a TiKV store disconnects or remains unavailable for a long time, the
pd/api/v1/stores response may return last_heartbeat_ts as nil.

The root cause is that watcher/meta synchronization can overwrite in-memory
store metadata with an incoming LastHeartbeat that is missing (0) or older,
which loses the previously observed latest heartbeat timestamp.

What is changed and how does it work?

core: preserve last heartbeat when syncing store meta

When store metadata is refreshed from watcher updates, incoming meta may miss
LastHeartbeat or carry an older value. This could overwrite in-memory store
metadata and make last_heartbeat_ts disappear in pd/api/v1/stores for
long-disconnected stores.

Keep LastHeartbeat monotonic in SetStoreMeta:
- preserve existing heartbeat when incoming value is zero/missing
- update only when incoming heartbeat is newer

Add regression test TestSetStoreMetaPreservesLastHeartbeat.

Changes in this PR:

1. Update core.SetStoreMeta to merge LastHeartbeat monotonically:
   • keep the current value by default;
   • only apply incoming LastHeartbeat when it is greater than the current one.
2. Add TestSetStoreMetaPreservesLastHeartbeat in pkg/core/store_test.go to
   verify preserve/update/non-regression behavior.

Check List

Tests

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

Code changes

• Has the configuration change
• Has HTTP APIs changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

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

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

Fix an issue where `last_heartbeat_ts` in `pd/api/v1/stores` could be lost
after long store disconnections.

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[ti-chi-bot]

Hi @ljluestc. Thanks for your PR.

I'm waiting for a tikv member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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