Add more server metrics for loop lag monitoring

[aylei]

Measures:

• DB histogram (ops, duration, percentiles) (not the SQL, just entire function)
• Background request execution histogram
• Event loop lag

test:

Draft dashboard json: SkyPilot API Server Edit-1756489275861.json

Next step:

• measure the sync block in async function
• measure the RSS and heap size of each process
• Update default dashboard

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[aylei]

/smoke-test --kubernetes

[aylei]

/smoke-test -k test_long_setup_run_script--kubernetes

[aylei]

/smoke-test -k test_long_setup_run_script --kubernetes

[SeungjinYang]

will approve once PR is out of draft

[SeungjinYang]

Thanks @aylei! LGTM