fix: Do not log /metrics requests from the pre-fork metrics server thread (Gunicorn deadlock)

[aborgatin]

What this PR does / why we need it:

The metrics HTTP server runs as a thread in the Gunicorn master, which forks the workers. Its default WSGIRequestHandler writes an access-log line to stderr for every scrape. A fork while that thread holds the stderr buffer lock leaves the new worker with the lock held forever: the worker blocks on its first log line ("Booting worker") and never serves a request. Details, stacks and a reproduction in #6928 (same class of problem as #6647).

This adds _QuietWSGIRequestHandler (no-op log_message, like prometheus_client's own _SilentHandler) and uses it for both the IPv4 and the dual-stack server built by _make_metrics_httpd. Scrapes are no longer written to stderr; nothing else changes.

Tested on Kubernetes with --max-requests 20 and a scrape storm: 5 hangs in 318 forks before, 0 hangs in 2,000 forks after. Locally (steps in #6928): a hang after 12–29 forks before, none in 336 forks after.

Which issue(s) this PR fixes:

Fixes #6928

Checks

• I've made sure the tests are passing. (pytest sdk/python/tests/unit/test_metrics.py: 117 passed; ruff check and ruff format --check pass; mypy feast reports no errors in metrics.py)
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests – TestMetricsHttpdDoesNotLog: a request to the server built by _make_metrics_httpd writes nothing to stderr (fails before the change, passes after)
• Integration tests

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.06%. Comparing base (8cfe891) to head (8e0f1e4).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6929   +/-   ##
=======================================
  Coverage   49.06%   49.06%           
=======================================
  Files         433      433           
  Lines       54314    54316    +2     
  Branches     7912     7912           
=======================================
+ Hits        26647    26649    +2     
  Misses      25788    25788           
  Partials     1879     1879           

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	50.45% <100.00%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
sdk/python/feast/metrics.py	81.81% <100.00%> (+0.17%)	⬆️

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[aborgatin]

@jyejare rebased onto master to pick up the pixi fix (#6947) – could you approve the workflow runs? Thanks!