feat: Bucket feature count labels in latency metrics

[jingqian-liu]

What this PR does / why we need it:

The feature_count label on
feast_feature_server_request_latency_seconds currently uses the exact
number of requested features. For workloads with widely varying feature
counts, this can create a large number of Prometheus time series and
increase metric cardinality.

This PR:

• Buckets feature_count label values using configurable boundaries.
• Adds metrics.feature_count_bins, defaulting to [10, 50, 200].
• Produces the default labels 0, 1-10, 11-50, 51-200, and 201+.
• Preserves 0 as a separate label for zero-feature requests.
• Keeps the exact feature count in audit logs; only the Prometheus latency
  metric label is bucketed.
• Validates that configured bucket boundaries are positive and strictly
  increasing.
• Documents the new configuration option.

For example:

feature_server:
  type: local
  metrics:
    enabled: true
    feature_count_bins: [5, 20]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.08%. Comparing base (2deea25) to head (8b13f85).
⚠️ Report is 1 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6811      +/-   ##
==========================================
- Coverage   47.08%   47.08%   -0.01%     
==========================================
  Files         419      419              
  Lines       51878    51876       -2     
  Branches     7525     7525              
==========================================
- Hits        24429    24427       -2     
  Misses      25700    25700              
  Partials     1749     1749              

Flag	Coverage Δ		*Carryforward flag
go-feature-server	30.58% <ø> (ø)
python-unit	48.39% <ø> (-0.01%)	⬇️	Carriedforward from 9b57eb5

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
sdk/python/feast/feature_server.py	62.84% <ø> (-0.06%)	⬇️
.../python/feast/infra/feature_servers/base_config.py	100.00% <ø> (ø)

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[jingqian-liu]

The macOS Python 3.12 unit-test job is still failing after the rerun. I checked the failure, and the only failing test is test_module_level_fips_sets_env_before_pyarrow_import, which times out after 60 seconds while importing feast.offline_server. All feature-server-related tests and the other Python/macOS/Linux unit-test jobs pass, so this appears unrelated to the changes in this PR.