*: add facility to print the iterator stack

[RaduBerinde]

To print the entire iterator stack tree, we can now use
fmt.Printf("%s", base.DebugTree(iter))
to get something like this:

*pebble.Iterator(0x14003a0ea08)
 ├── *keyspan.InterleavingIter(0x14003ee2030)
 │    ├── *invalidating.iter(0x14007c52f00)
 │    │    └── *pebble.mergingIter(0x14003a0f028)
 │    │         └── *pebble.levelIter(0x14003a0f3a0) L6: fileNum=001316
 │    │              └── *sstable.singleLevelIterator(0x14004bce008) fileNum=001316
 │    └── *keyspan.assertIter(0x14001229950)
 │         └── *keyspan.DefragmentingIter(0x14003ee26e8)
 │              └── *keyspan.BoundedIter(0x14003ee2678)
 │                   └── *keyspanimpl.MergingIter(0x14003ee2300)
 │                        └── *keyspanimpl.LevelIter(0x14003ee2788) L6
 │                             └── *keyspan.assertIter(0x14000753a70)
 │                                  └── *sstable.fragmentBlockIter(0x140000ffb88) fileNum=001316
 └── *invalidating.iter(0x14007c52f00)
      └── *pebble.mergingIter(0x14003a0f028)
           └── *pebble.levelIter(0x14003a0f3a0) L6: fileNum=001316
                └── *sstable.singleLevelIterator(0x14004bce008) fileNum=001316

[cockroach-teamcity]

This change is 

[nicktrav]

This is really nice! A couple comments.

Any way to annotate the level iters with level numbers? There's possibly other metadata that might be useful too? Position? Direction? Data iterated over so far, etc?

Could this then be useful up in Cockroach, say, for trace spans - we can print this debug stack. It could supplement the iterator stats nicely.

[RaduBerinde]

TFTR!

I added the level for level iterators and filenum for "leaf" iterators.

I'm not sure right now how to use it during tracing, since it depends when you look at the tree. I think if we add more information, printing it at the end might be useful.

*pebble.Iterator(0x14003a0ea08)
 ├── *keyspan.InterleavingIter(0x14003ee2030)
 │    ├── *invalidating.iter(0x14007c52f00)
 │    │    └── *pebble.mergingIter(0x14003a0f028)
 │    │         └── *pebble.levelIter(0x14003a0f3a0) L6: fileNum=001316
 │    │              └── *sstable.singleLevelIterator(0x14004bce008) fileNum=001316
 │    └── *keyspan.assertIter(0x14001229950)
 │         └── *keyspan.DefragmentingIter(0x14003ee26e8)
 │              └── *keyspan.BoundedIter(0x14003ee2678)
 │                   └── *keyspanimpl.MergingIter(0x14003ee2300)
 │                        └── *keyspanimpl.LevelIter(0x14003ee2788) L6
 │                             └── *keyspan.assertIter(0x14000753a70)
 │                                  └── *sstable.fragmentBlockIter(0x140000ffb88) fileNum=001316
 └── *invalidating.iter(0x14007c52f00)
      └── *pebble.mergingIter(0x14003a0f028)
           └── *pebble.levelIter(0x14003a0f3a0) L6: fileNum=001316
                └── *sstable.singleLevelIterator(0x14004bce008) fileNum=001316

Reviewable status: 0 of 30 files reviewed, all discussions resolved (waiting on @jbowens)

[RaduBerinde]

Ok to merge? This has been useful when debugging some meta failures.

[RaduBerinde]

Friendly ping

[jbowens]

Reviewed 4 of 30 files at r1, 2 of 19 files at r2, 3 of 15 files at r3.
Reviewable status: 9 of 34 files reviewed, all discussions resolved

[RaduBerinde]

TFTR!