sql: expose feature counters via crdb_internal.feature_usage

[knz]

First 3 commits from #31332.
Only last commit in this PR; will rebase when the other is merged.

I found myself using this extensively to work on that other PR and related PRs adding markers for unimplemented features and new feature counts.

[cockroach-teamcity]

This change is 

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[knz]

cc @dt

[knz]

cc @dt

[bobvawter]

Reviewed 1 of 1 files at r1, 7 of 7 files at r2, 5 of 5 files at r3, 6 of 6 files at r4.
Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale)

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pkg/server/telemetry/features.go, line 96 at r2 (raw file):

// inspection via SQL. They are not quantized! Thus not suitable for
// reporting.
func GetFeatureCounts() map[string]int32 {

Maybe GetFeatureCountsForDisplay() to make the intended use more obvious if you don't read the doc?

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pkg/sql/logictest/testdata/logic_test/crdb_internal, line 140 at r4 (raw file):

query TT colnames
SELECT * FROM crdb_internal.feature_usage WHERE feature_name = ''

Might it make sense to select count(usage_count) from feature_usage where feature_name = 'something' yields 1 to ensure that the wiring from telemetry to the virtual table is functioning? I see why you wouldn't want to query for a specific value, but this doesn't verify that the populate struct member is actually populating anything.