*: reduce CPU overhead on hot paths

[lidezhu]

What problem does this PR solve?

Issue Number: close #5107

What is changed and how it works?

Before this pr:

After this pr:

Check List

Tests

• Unit test
• Integration test
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• No code

Questions

Will it cause performance regression or break compatibility?

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Release note

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📥 Commits

Reviewing files that changed from the base of the PR and between 00684df and 23b67df.

📒 Files selected for processing (5)

• downstreamadapter/eventcollector/dispatcher_stat.go
• downstreamadapter/eventcollector/helper.go
• logservice/logpuller/region_event_handler.go
• utils/dynstream/stream.go
• utils/dynstream/stream_test.go

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[gemini-code-assist]

Code Review

This pull request focuses on performance optimizations by reducing overhead in logging and metrics collection. It introduces explicit log level checks before debug statements to prevent unnecessary field evaluation and implements caching for Prometheus metric observers to avoid the cost of WithLabelValues in performance-critical paths. A review comment suggests simplifying a switch statement in region_event_handler.go by removing a redundant case that re-assigns a default value.

[wuhuizuo]

/test ?

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