Added metrics related to EMQX durable storage to Prometheus.

New metrics:

- `emqx_ds_egress_batches`

- `emqx_ds_egress_batches_retry`

- `emqx_ds_egress_batches_failed`

- `emqx_ds_egress_messages`

- `emqx_ds_egress_bytes`

- `emqx_ds_egress_flush_time`

- `emqx_ds_store_batch_time`

- `emqx_ds_builtin_next_time`

- `emqx_ds_storage_bitfield_lts_counter_seek`

- `emqx_ds_storage_bitfield_lts_counter_next`

- `emqx_ds_storage_bitfield_lts_counter_collision`

Note: these metrics are only visible when session persistence is enabled.

Number of persisted messages has been also added to the dashboard.