db: add metric encapsulating blob file rewrite bytes read, written

[annrpom]

Blob file rewrite compactions happen independent of LSM levels, and so don't
contribute to the per-level 'compacted bytes' total. This patch adds separate
metrics recording the bytes read and bytes written during these compactions.

Fixes: #5444

[cockroach-teamcity]

This change is 

[sumeerbhola]

@sumeerbhola reviewed 8 of 9 files at r1.
Reviewable status: 8 of 9 files reviewed, 3 unresolved discussions

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-- commits line 4 at r1:
independent of the level?

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testdata/compaction/value_separation line 546 at r1 (raw file):

    est. debt |   in progress |  cancelled |   failed |    problem spans |       read |    written
--------------+---------------+------------+----------+------------------+------------+-----------
           0B |        0 (0B) |     0 (0B) |        0 |                0 |        83B |       150B

why is the read smaller than the written?

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blob_rewrite.go line 251 at r1 (raw file):

		// rewrites don't contribute to per-level compacted bytes.
		// Count blob value blocks read from the blob file during the rewrite.
		bytesRead := c.internalIteratorStats.BlockReads[blockkind.BlobValue].BlockBytes

Is this why we bytes read could be smaller, because we don't count the various meta blocks or properties etc., while the write path does? Which is fine.

[annrpom]

Reviewable status: 8 of 9 files reviewed, 3 unresolved discussions (waiting on @sumeerbhola)

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-- commits line 4 at r1:

Previously, sumeerbhola wrote…

independent of the level?

yes; corrected wording!

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blob_rewrite.go line 251 at r1 (raw file):

Previously, sumeerbhola wrote…

Is this why we bytes read could be smaller, because we don't count the various meta blocks or properties etc., while the write path does? Which is fine.

yes - i'll add a comment for this as well

[sumeerbhola]

@sumeerbhola reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @annrpom)

[annrpom]

TFTR! ('-')7