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 47.44%. Comparing base (640ac70) to head (7107102).
⚠️ Report is 1 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6811      +/-   ##
==========================================
+ Coverage   47.41%   47.44%   +0.02%     
==========================================
  Files         421      421              
  Lines       52249    52273      +24     
  Branches     7582     7587       +5     
==========================================
+ Hits        24776    24801      +25     
  Misses      25708    25708              
+ Partials     1765     1764       -1     

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	48.77% <100.00%> (+0.02%)	⬆️

Files with missing lines	Coverage Δ
sdk/python/feast/feature_server.py	63.63% <100.00%> (+0.71%)	⬆️
.../python/feast/infra/feature_servers/base_config.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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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.

[jingqian-liu]

Rebased onto current master to pick up #6835, which fixes the CPU wheel hash mismatch and stabilizes the FIPS import test seen in the previous CI runs. CI is rerunning now. Thanks!

[ntkathole]

@jingqian-liu can you please resolve conflicts ?

[jingqian-liu]

@jingqian-liu can you please resolve conflicts ?

Resolved the conflicts with current master and pushed the updated branch. Thanks!

[jingqian-liu]

@jingqian-liu can you please resolve conflicts ?

Hi @ntkathole, the conflicts are resolved and the branch has been rebased onto current master. Since master is moving quickly, if everything looks good after the workflows are approved, would you mind merging this when convenient? Thanks!