Add metrics.disaggStorage.logicalSize.bytesOplog: the logical BSON size of local.oplog.rs.

The existing bytesHot metric continues to include the oplog. Exposing the component separately lets Atlas display how oplog retention contributes to a customer's storage usage and billing.

Acceptance criteria

• Export bytesOplog through serverStatus and OpenTelemetry.

• Allowlist the serverStatus field for customer access.

• bytesOplog increases with oplog growth and decreases after truncation.

• bytesOplog is included in bytesHot.

• Add unit and integration test coverage.

Context

The product decision is to meter the oplog collection and display its size separately. bytesHot remains the backwards-compatible aggregate.