fix(io-metrics): drop client-controlled bucket label from rustfs_s3_operations_total (backlog#806-22)
The bucket dimension on the rustfs_s3_operations_total counter is
client-controlled and unbounded, letting any client explode metric
cardinality (a Prometheus/OTEL cardinality DoS that grows the in-process
OTEL aggregation store even without a scrape). Drop the bucket label so the
series is keyed by the bounded op dimension only (<=122 variants), matching
MinIO which never labels its default operation counters with bucket.
BREAKING: rustfs_s3_operations_total no longer carries a "bucket" label.
record_s3_op(op, bucket) -> record_s3_op(op); all call sites in
rustfs/src/storage/{ecfs.rs,helper.rs} updated (bucket bindings retained
where still used by audit/notify paths). Add metrics-util debugging recorder
dev-dep and tests asserting the series carries only the op label and that
series count equals distinct-op count.
Zhengchao An