[Dashboard] Optimizing performance of Ray Dashboard

[alexeykudinkin]

Why are these changes needed?

• Reduced & aligned intervals updating node's workers stats to 15s
• Revisited node stats updating seq to poll raylets in chunks (of no more than 100 at a time)
• Revisit ActorHead APIs to allow fetching individual actor w/o the need to process every actor ever-created
• Remove singleton (single-threaded) TPE in both Head/Agents with standalone (single-threaded) TPEs in every Head/Agent
• Added metrics for # of tasks running on Dashboard's EVL
• Misc fixes

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[alexeykudinkin]

@alanwguo if we want to preserve UI update frequency to 2s we'll need to switch to pulling model in the UI

[alanwguo]

I think 15s is probably fine.

[alanwguo]

I think 15s is probably fine.

[alexeykudinkin]

For those who might run into this in the future: CI is failing b/c i'm adding module named collections which apparently (based on just the filename) collides with system one (sic!)

/rayci/python/ray/_private/log_monitor.py:26: SyntaxWarning: invalid escape sequence '\d'
/rayci/python/ray/_private/log_monitor.py:28: SyntaxWarning: invalid escape sequence '\d'
Traceback (most recent call last):
  File "/rayci/python/ray/_private/log_monitor.py", line 1, in <module>
    import argparse
  File "/opt/miniconda/lib/python3.12/argparse.py", line 89, in <module>
    import re as _re
  File "/opt/miniconda/lib/python3.12/re/__init__.py", line 124, in <module>
    import enum
  File "/opt/miniconda/lib/python3.12/enum.py", line 5, in <module>
    from functools import reduce
  File "/opt/miniconda/lib/python3.12/functools.py", line 18, in <module>
    from collections import namedtuple
  File "/rayci/python/ray/_private/collections.py", line 1, in <module>
    from typing import List, Any
  File "/opt/miniconda/lib/python3.12/typing.py", line 24, in <module>
    from collections import defaultdict
ImportError: cannot import name 'defaultdict' from partially initialized module 'collections' (most likely due to a circular import) (/rayci/python/ray/_private/collections.py)