Add write/read bandwidth info of a shard to special key space metrics module

Now in special key space, through
```
>>> for k, v in db.get_range_startswith('\xff\xff/metrics/data_distribution_stats/', limit=3):
... print(k, v)
...
('\xff\xff/metrics/data_distribution_stats/', '{"shard_bytes":3828000}')
('\xff\xff/metrics/data_distribution_stats/mako00079', '{"shard_bytes":2013000}')
('\xff\xff/metrics/data_distribution_stats/mako00126', '{"shard_bytes":3201000}')
```
We can just get the shard estimated size. Since we have some shard rebalance and shard merge/split action also relies on write/read bandwidth, it's helpful to add these info for better DD observability.

Start in the special key space metrics module and inspect the existing data_distribution_stats entries, including the shard_bytes values shown in the issue. Determine how write and read bandwidth should be represented alongside shard size, and consider the rebalance and shard merge/split consumers as the completion criteria for the observability data.

Written by the indexing model from the issue text.