LU-15564 osd: add allocation time histogram

LU-15564 osd: add allocation time histogram

add block mapping/allocation histogram to brw stats to debug
mballoc related issues.

$ lctl get_param osd*.*OST*.brw_stats
                           read      |     write
block maps msec        maps  % cum % |  maps        % cum %
1:		   1522360 100 100   | 49272  99  99
2:		         0   0 100   |    1   0  99
4:		         0   0 100   |    1   0  99
8:		         0   0 100   |    0   0  99
16:		         0   0 100   |    0   0  99
32:		         0   0 100   |    0   0  99
64:		         0   0 100   |    1   0 100

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I1185386adc64e844de71e25a4e439e493e5e5bc5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/46550
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>