Overview

IncrementalRolloutFeatureFlag::getUnknownWireFlagsDroppedCount() (a process-wide counter) and IncrementalFeatureRolloutContext::getUnknownWireFlags() (a per-context set of unrecognized flag names) are both populated today by IncrementalFeatureRolloutContext::fromWire's unknown-flag path, but neither is currently surfaced through serverStatus or FTDC. There is no production reader of either value yet.

Background

SERVER-130023 added these diagnostics to help detect mixed-version clusters sending IFR flag names a given binary doesn't recognize — typically benign during a rolling upgrade, but worth alerting on if persistent or unexpected. See PR #57081, src/mongo/db/feature_flag.h/feature_flag.cpp.

Scope of Work

• Wire IncrementalRolloutFeatureFlag::getUnknownWireFlagsDroppedCount() into the existing IFR serverStatus section (see IncrementalRolloutFeatureFlag::appendFlagsStats) or a new top-level counter.
• Decide whether/how per-request getUnknownWireFlags() names should be surfaced (e.g. FTDC, a capped in-memory log, or a diagnostic command) without unbounded cardinality risk.

Acceptance Criteria

• An operator can observe, via serverStatus or FTDC, whether/how often this binary is dropping unrecognized IFR flags received on the wire.