Observer: add some metrics for the waiting queue of snapshot generator.
What is changed and how it works?
Issue Number: Close #12945
What's Changed:
Related changes
Check List
Tests
- Manual test (add detailed scripts or steps below)

Side effects
Release note
None.
[REVIEW NOTIFICATION]
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- Connor1996
- lhy1024
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/cc @lhy1024 @AndreMouche
PTAL @AndreMouche again
PTAL @Connor1996
/ping @nolouch
/ping @Connor1996 , help me merge it
/merge
@AndreMouche: It seems you want to merge this PR, I will help you trigger all the tests:
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This pull request has been accepted and is ready to merge.
@bufferflies: Your PR was out of date, I have automatically updated it for you.
At the same time I will also trigger all tests for you:
/run-all-tests
If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.
Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.