sstable: include blocks other than point key blocks in iterator stats

[nicktrav]

The base.InternalIteratorStats struct maintains stats on the bytes read from SSTable blocks while iterating, including whether the block was found in the block cache or not.

Currently, these stats only take point key data blocks into account. SSTables contains other types of blocks that must be loaded during iteration. For example, range del or range key blocks. If a range del block is large, and uncached, the current results can be misleading (see cockroachlabs/support#2203 for an example).

Take these other blocks into account in the iterator statistics.

Jira issue: PEBBLE-197

[arthurgavazza]

hey @nicktrav I wanna start contributing to pebble and I'm looking for simple issues to work on, do you think this one is accessible? If I pick this up can you give me a few tips on where to start?

[sumeerbhola]

We also have BlockReadDuration that suffers from the same issue.

And, in addition to the blocks read by the iterator, there are the blocks read when the Reader is created (in case it was created due to an iterator), including the footer, properties, metaindex. Ideally, these should also be in these stats, though if tracing is enabled we may be able to manage without once #3728 is fixed.

[jbowens]

I'm anticipating that Edward will pick this up as a starter project.