Have you tried Badger before this proposal? and did not find anything similar?

Yes

What you wanted to do.

Get total size of keys listed by badger info --show-keys --with-prefix ....

What you actually did.

I had to make a script to parse the output and compute the sum from the size of each single item.

Why wasn't it great, with examples.

The sub-command already returns the total number of keys counting listed keys. It also returns the size of each key-value pair listed. The total size seems like a missing part which completes the picture.

Additional information.

After the feature implementation badger info output may look like the following (new line starts with Total Size...).

$ badger info --show-keys --with-prefix 0815 --dir .
Listening for /debug HTTP requests at port: 8080

...

[Summary]
Total Number of keys: 3908
Total Size of key-value pairs: 1290200
Num Allocated Bytes at program end: 0 B