chore(debug): add summary to show LSM tree and namespace size (#7891)

[all-seeing-code]

Add --only-summary flag in dgraph debug to show LSM tree and namespace sizes.

(cherry picked from commit cc87153)

The flag restricts the output to only the summary:

╰─➤  dgraph debug --postings ./p --only-summary

Opening DB: ./p
Listening for /debug HTTP requests at port: 8080
badger 2022/12/23 01:01:08 INFO: All 1 tables opened in 0s
badger 2022/12/23 01:01:08 INFO: Discard stats nextEmptySlot: 0
badger 2022/12/23 01:01:08 INFO: Set nextTxnTs to 11
[SUMMARY]
Level 0 size:      1.3 KiB
Level 1 size:          0 B
Level 2 size:          0 B
Level 3 size:          0 B
Level 4 size:          0 B
Level 5 size:          0 B
Level 6 size:          0 B
Total SST size:      1.3 KiB

Namespace 0x0 size:      1.3 KiB (banned)

badger 2022/12/23 01:01:08 INFO: Lifetime L0 stalled for: 0s
badger 2022/12/23 01:01:08 INFO: 
Level 0 [ ]: NumTables: 01. Size: 1.3 KiB of 0 B. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 64 MiB
Level 1 [ ]: NumTables: 00. Size: 0 B of 10 MiB. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 2.0 MiB
Level 2 [ ]: NumTables: 00. Size: 0 B of 10 MiB. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 2.0 MiB
Level 3 [ ]: NumTables: 00. Size: 0 B of 10 MiB. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 2.0 MiB
Level 4 [ ]: NumTables: 00. Size: 0 B of 10 MiB. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 2.0 MiB
Level 5 [ ]: NumTables: 00. Size: 0 B of 10 MiB. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 2.0 MiB
Level 6 [B]: NumTables: 00. Size: 0 B of 10 MiB. Score: 0.00->0.00 StaleData: 0 B Target FileSize: 2.0 MiB
Level Done

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ anurags92
❌ NamanJain8
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[all-seeing-code]

@mangalaman93 This is a pretty benign change, I am wondering if we need tests for this too. I don't think debug tool is tested right now.

[mangalaman93]

what kind of tests would even be useful for this change?

[coveralls]

Coverage: 64.388% (-0.01%) from 64.401% when pulling 6639a11 on anurag/cherry-pick-lsm-summary into 4c1b575 on main.