Add description for each metric

[sdwerwed]

Hello,

I see in general some confusion about what those metrics really monitor, can we add a description on those if there is there no documentation for it?

fluentd_status_buffer_queue_length
fluentd_status_buffer_total_bytes
fluentd_status_retry_count
fluentd_status_buffer_newest_timekey
fluentd_status_buffer_oldest_timekey

fluentd_output_status_retry_count
fluentd_output_status_num_errors
fluentd_output_status_emit_count
fluentd_output_status_retry_wait
fluentd_output_status_emit_records
fluentd_output_status_write_count
fluentd_output_status_rollback_count
fluentd_output_status_flush_time_count
fluentd_output_status_slow_flush_count

fluentd_output_status_buffer_total_bytes
fluentd_output_status_buffer_stage_length
fluentd_output_status_buffer_stage_byte_size
fluentd_output_status_buffer_queue_length
fluentd_output_status_buffer_queue_byte_size
fluentd_output_status_buffer_newest_timekey
fluentd_output_status_buffer_oldest_timekey
fluentd_output_status_buffer_available_space_ratio fro