Fix dict hanlding in pool and metrics

[dkropachev]

There are certain rules you need to following in the cases when dicts are modified and read in parallel.
Otherwise you end up with RuntimeError: dictionary changed size during iteration. Rules are the following:

1. Any iteration over items or keys, needs to be done over a snapshot, i.e. list() or set()
2. Avoid unnecessary iterations like len(d.keys()), you can replace them with len(d)

This commit fixes code to match these rules in the pool.py and metrics.py

Fixes: #594

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[mykaul]

Typo in commit title, otherwise looks OK.

[dkropachev]

Typo in commit title, otherwise looks OK.

Fixed, pipeline is fialing because of the weird network issue:

 Trying to resolve the latest version from remote
  (node:2094) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
  (Use `node --trace-deprecation ...` to show where the warning was created)
  Error: Java setup failed due to network issue or timeout: error code: 500
  Error: error code: 500

No changes to the code done since last cicd run.

[fruch]

I would guess cloudflare issues, half of the internet was broken today cause of those

Typo in commit title, otherwise looks OK.

Fixed, pipeline is fialing because of the weird network issue:

 Trying to resolve the latest version from remote
  (node:2094) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
  (Use `node --trace-deprecation ...` to show where the warning was created)
  Error: Java setup failed due to network issue or timeout: error code: 500
  Error: error code: 500

No changes to the code done since last cicd run.