What this PR does / why we need it:

Reduces per-request overhead on the /get-online-features endpoint that was
introduced between v0.61.0 and v0.65.0. Users upgrading across this range
observed p99 latency increase from ~80 ms to ~200 ms with the same
worker/replica configuration.

Two root causes are addressed:

1. Count PRESENT statuses incrementally (utils.py)

track_feature_statuses() (added in #6280) scans the entire feat_statuses
matrix after it is already built:

# Before — O(n_features × output_len) re-scan
_present = sum(s == PRESENT for row in feat_statuses for s in row)

For a request with 25 features × 100 entities × 3 feature views, this
re-scans 7,500 status values. This PR counts PRESENT values during the
existing assignment loop, making the counting O(1) additional work:

# After — incremented during the existing loop
_present_count += 1

2. Skip audit overhead when audit_logging is disabled (feature_server.py)

_emit_online_audit() (added in #6340) runs in the finally block of every
online request. Even with audit_logging: false (the default), every request
still:

• Calls time.monotonic() twice for audit timing
• Calls get_security_manager() to resolve requestor identity
• Calls _parse_feature_info() to re-parse the feature list

This PR adds an early-exit guard on feast_metrics._config.audit_logging so
that when audit logging is disabled, none of this work is performed.