Add log_item/log_byte reporting and client report docs updates

[szokeasaurusrex]

Part of #882. Depends on #1003 and #1004.

Ensure dropped logs always record both log_item (count) and log_byte (estimated bytes) outcomes, using a reusable size estimation helper across log drop paths. Update README/docs for send_client_reports, including the self-hosted minimum version (21.9.0) and explicit guidance to set send_client_reports = false on older self-hosted deployments; add integration coverage for log client outcomes.

[linear]

RUST-157