MongoDB Exporter does not expose fsyncLock (`db.fsyncLock`) metrics

The Percona MongoDB Exporter currently does not expose any metric related to the MongoDB fsyncLock state (i.e., when the server is locked using `db.fsyncLock()` for backup operations). This makes it difficult to monitor or alert on situations where a MongoDB instance is stuck in a locked state or backups leave the node locked longer than expected. Expected Behavior: - The exporter should expose one or more Prometheus metrics indicating: - Whether the server is currently fsync-locked - The lock count returned by the `fsync` command - The duration the instance has been in fsyncLock (optional but useful) Example potential metrics: ``` mongodb_fsync_lock_state (0/1) mongodb_fsync_lock_count mongodb_fsync_lock_seconds ``` **Why This Matters** - Backups that rely on db.fsyncLock() can leave a node in a locked state if something fails. - Operators currently have no way to detect this via Prometheus/Grafana. - Monitoring this state is critical for: - Backup automation - Cluster availability - Detecting issues on MongoDB secondaries used for backups - Preventing prolonged write blocking due to accidental fsync-lock **Suggested Solution** Add support in the exporter to read and expose `fsyncLock` values from MongoDB diagnostic output by: 1. Capturing the fsyncLock / lockCount returned by MongoDB admin commands. 2. Lock state command `db.adminCommand({ currentOp: 1 }).fsyncLock` 3. Mapping these fields to Prometheus metrics under a consistent naming scheme. Happy to help test or validate the feature if added.