db: add initFileOpts for initializing in the FileCache

[sumeerbhola]

initFileOpts only contains InitFileReadStats, which allows the caller to provide stats collected by iterators. Reads done when creating a Reader specifically because an iterator was needed on that file, will now be captured in the corresponding iterator stats.

This reduces the uncertainty when we see slowness in traces around iteration, and are unsure whether it happened prior to operations on the iterator. We continue to have some unaccounted latency:

• Footer reads are not included in the stats.
• File cache entry initialization is shared when there are concurrent attempts to create the entry, and the latency (via iterator stats) is only accounted in the one that did the Reader creation.

initFileOpts are also plumbed to valblk.blockProviderWhenClosed, when
it needs to open a sstable.Reader. As part of this change, we also
fix the missing stats for valblk.ReadValueBlockExternal, and the
missing category stats in general when reading value blocks.

[cockroach-teamcity]

This change is 

[sumeerbhola]

also related to https://cockroachlabs.slack.com/archives/C04U1BTF8/p1754564947355449

Reviewable status: 0 of 15 files reviewed, all discussions resolved (waiting on @RaduBerinde)

[jbowens]

Reviewed 13 of 15 files at r1, all commit messages.
Reviewable status: 13 of 15 files reviewed, 2 unresolved discussions (waiting on @RaduBerinde)

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file_cache.go line 283 at r1 (raw file):

		fileType: ftyp,
	}
	valRef, err := h.fileCache.c.FindOrCreate(ctx, key, initFileOpts{})

relates to my other question, but should we collect these stats for blob file opens too--or leave a TODO to do so?

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sstable/options.go line 138 at r1 (raw file):

// and we are able to capture these initialization stats in the iterator
// stats.
type InitFileReadStats struct {

should we put this in the block package and collect it for blob file opens too?

[sumeerbhola]

TFTR!

Reviewable status: 9 of 22 files reviewed, 2 unresolved discussions (waiting on @jbowens and @RaduBerinde)

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file_cache.go line 283 at r1 (raw file):

Previously, jbowens (Jackson Owens) wrote…

relates to my other question, but should we collect these stats for blob file opens too--or leave a TODO to do so?

I had the TODO earlier about collecting stats for blob file opens, but looking at it again, that was for the last step and the first step, but there is no reason to not put in most of the plumbing now, and just leave the last step as a TODO.
Fixed now.

Also made some other plumbing improvements in valblk related to reading blocks.

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sstable/options.go line 138 at r1 (raw file):

Previously, jbowens (Jackson Owens) wrote…

should we put this in the block package and collect it for blob file opens too?

Done

[jbowens]

Reviewed 11 of 11 files at r2, all commit messages.
Reviewable status: 20 of 22 files reviewed, 2 unresolved discussions (waiting on @RaduBerinde)