Refactor: introduce a new metrics for cdc_backfill

[fuyufjh]

risingwave/src/stream/src/executor/backfill/cdc_backfill.rs

Lines 319 to 325 in f2e79d3

	self.metrics
	.backfill_snapshot_read_row_count
	.with_label_values(&[
	upstream_table_id.to_string().as_str(),
	self.actor_id.to_string().as_str(),
	])
	.inc_by(cur_barrier_snapshot_processed_rows);

Currently, the cdc_backfill executor reuses the streaming backfill's metrics. This looks weird. Please introduce a set of new metrics, such as

    pub cdc_backfill_snapshot_read_row_count: GenericCounterVec<AtomicU64>,
    pub cdc_backfill_upstream_output_row_count: GenericCounterVec<AtomicU64>,