[Enhancement] add running slow query metric

[pengliang100]

Why I'm doing:

Add the starrocks_fe_running_slow_query metric, which allows monitoring and alerting when slow queries occur in the cluster

What I'm doing:

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.0
  • 3.5
  • 3.4
  • 3.3

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Note

Adds a Frontend gauge to expose the number of currently running slow queries.

• New running_slow_query gauge in MetricRepo registers the live count of slow-running queries
• Implements QeProcessorImpl.runningSlowQueries() that counts coordinators exceeding Config.qe_slow_log_ms

^{Written by Cursor Bugbot for commit 4656b1a. This will update automatically on new commits. Configure here.}

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[alvin-celerdata]

@cursor review