add bucket sharks and obj nums metrics support

[oneto1]

Hi , can we add some new bucket metrics ?

When num_objects and num_objects is too big , maybe it will be a performance problem about rgw .

radosgw-admin bucket stats --bucket p
{
    "bucket": "p",
    "num_shards": 64,   <<<<<<<<<
    "tenant": "", 
    ...... 
    "usage": {
        "rgw.main": {
            "size": 530835773,
            "size_actual": 18685812736,
            "size_utilized": 530835773,
            "size_kb": 518395,
            "size_kb_actual": 18247864,
            "size_kb_utilized": 518395,
            "num_objects": 4461780.  <<<<<<<<<
        },
}

Add those to help monitor ceph(rgw) better.

[baergj]

👋 This could maybe be added behind a enable/disable flag, as the cardinality of this data can get rather high.

It's on the list for now. Patches welcome, as always!