Consider adding the stats of region data size

[WenyXu]

          Yep, it only contains the manifest size, can it include the data size too?

Originally posted by @killme2008 in #4070 (comment)

[evenyag]

We can collect the file sizes from the SstVersion

greptimedb/src/mito2/src/sst/version.rs

Lines 87 to 102 in 0575108

	/// Returns SST files'space occupied in current version.
	pub(crate) fn sst_usage(&self) -> u64 {
	self.levels
	.iter()
	.map(|level_meta| {
	level_meta
	.files
	.values()
	.map(|file_handle| {
	let meta = file_handle.meta_ref();
	meta.file_size + meta.index_file_size
	})
	.sum::<u64>()
	})
	.sum()
	}

[killme2008]

Looks like already exists:

greptimedb/src/mito2/src/region.rs

Line 48 in df0fff2

pub struct RegionUsage {

greptimedb/src/mito2/src/engine.rs

Line 127 in df0fff2

pub fn get_region_usage(&self, region_id: RegionId) -> Result<RegionUsage> {