feat: Make feature-count latency histogram bins configurable #6640

Follow-up from #5920 (Improve Feature Server Observability).

Problem

request_latency in sdk/python/feast/metrics.py is labeled with feature_count using the raw count of features requested (see track_request_latency / RequestMetricsContext.feature_count). Raw counts create unbounded label cardinality — every distinct feature count seen in production becomes its own time series. The issue asks for configurable bins instead, e.g. 1-10, 11-50, 51-200, 201+.

Proposed solution

• Add a binning function (e.g. bin_feature_count(count: int, bins: list[int]) -> str) that maps a raw feature count to a bin label like "1-10", "11-50", "51-200", "201+".
• Default bins should match the issue's suggestion (1-10, 11-50, 51-200, 201+) but should be configurable via MetricsConfig in feature_store.yaml (e.g. metrics.feature_count_bins: [10, 50, 200]).
• Apply this wherever feature_count is currently set as a raw value before being passed into RequestMetricsContext in sdk/python/feast/feature_server.py.
• Same treatment could apply to feature_view_count.
• Update relevant docs (docs/getting-started/components/feature-server.md or wherever metrics config is documented) to describe the new config option.

cc: @jyejare @ntkathole