In SERVER-44539 we re-enable the missing RWC logging. As a follow-up, we should evaluate wether or not to add readCornern logging for non-transactional external requests without a readConcern or a custom cluster-wide default (referencing this PR comment). In this case, _extractReadConcern() early returns  without reaching the check on the presence of a read concern, including on shard/config servers. Missing readConcern therefore remains unlogged for otherwise eligible commands in that configuration.

As part of this ticket, we should evaluate wether to make the fast path emit the same rate-limited diagnostic while preserving the existing eligibility checks and cache-lookup optimization, and add coverage without a custom default.