Partially covers #5630

Add `pipeline_dispatch_flush_duration_seconds` metrics that bridge the gap
between pipeline_commands_duration_seconds and cmd_squash_hop_duration_seconds + pipeline_queue_wait_duration_seconds.

In summary,
pipeline_commands_duration_seconds roughly equals to `dragonfly_cmd_squash_hop_duration_seconds + cmd_squash_hop_duration_seconds + pipeline_queue_wait_duration_seconds`.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>