pkg/util/span: potential performance regression

pkg/util/span [performance regression](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Nightlies_RoachprodMicrobenchmarkWeeklyWip/21021417?buildTab=log) on master @ [867004a4311025b7e0de44bc8c72fcbc8442f041](https://github.com/cockroachdb/cockroach/commits/867004a4311025b7e0de44bc8c72fcbc8442f041):

Performance regressions detected in package pkg/util

Comparison: v25.4.3 (71d85362) -> refs/heads/master (867004a4)

Found 24 benchmark(s) with regressions ≥20%:

- [ ] [pkg/util/span→BTreeIterSeekLT/count=65536-32 (sec/op): +43.48% (43.5%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeIterSeekLT/count=65536-32&package=pkg/util/span)
- [ ] [pkg/util/syncutil→LoadMostlyHits/*syncutil.DeepCopyMap-32 (sec/op): +40.05% (40.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=LoadMostlyHits/*syncutil.DeepCopyMap-32&package=pkg/util/syncutil)
- [ ] [pkg/util/interval/generic→BTreeIterSeekLT/count=65536-32 (sec/op): +39.29% (39.3%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeIterSeekLT/count=65536-32&package=pkg/util/interval/generic)
- [ ] [pkg/util/interval→BTreeInsert/count=65536-32 (sec/op): +37.98% (38.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeInsert/count=65536-32&package=pkg/util/interval)
- [ ] [pkg/util/span→BTreeDelete/count=65536-32 (sec/op): +32.04% (32.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeDelete/count=65536-32&package=pkg/util/span)
- [ ] [pkg/util/interval/generic→BTreeDelete/count=65536-32 (sec/op): +31.56% (31.6%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeDelete/count=65536-32&package=pkg/util/interval/generic)
- [ ] [pkg/util/quotapool→ConcurrentIntPool/workers=512,quota=511-32 (sec/op): +30.85% (30.9%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ConcurrentIntPool/workers=512,quota=511-32&package=pkg/util/quotapool)
- [ ] [pkg/util/span→BTreeDeleteInsertCloneEachTime/count=65536/reset=true-32 (sec/op): +30.30% (30.3%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeDeleteInsertCloneEachTime/count=65536/reset=true-32&package=pkg/util/span)
- [ ] [pkg/util/quotapool→ConcurrentIntPool/workers=512,quota=513-32 (sec/op): +26.68% (26.7%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ConcurrentIntPool/workers=512,quota=513-32&package=pkg/util/quotapool)
- [ ] [pkg/util/log→ExpensiveLogEnabled-32 (sec/op): +26.48% (26.5%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ExpensiveLogEnabled-32&package=pkg/util/log)
- [ ] [pkg/util/interval/generic→BTreeDeleteInsertCloneEachTime/count=65536/reset=true-32 (sec/op): +25.38% (25.4%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeDeleteInsertCloneEachTime/count=65536/reset=true-32&package=pkg/util/interval/generic)
- [ ] [pkg/util/tracing→SpanCreation/detached-child=false-32 (sec/op): +23.60% (23.6%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=SpanCreation/detached-child=false-32&package=pkg/util/tracing)
- [ ] [pkg/util/syncutil→AdversarialAlloc/*syncutil.RWMutexMap-32 (sec/op): +22.71% (22.7%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=AdversarialAlloc/*syncutil.RWMutexMap-32&package=pkg/util/syncutil)
- [ ] [pkg/util/encoding→PeekType-32 (sec/op): +20.16% (20.2%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=PeekType-32&package=pkg/util/encoding)
- [ ] [pkg/util→FastIntMap/10x10-4/map-sized-32 (allocs/op): +200.00% (200.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/10x10-4/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/32x15-10/map-32 (allocs/op): +200.00% (200.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/32x15-10/map-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/32x15-10/map-sized-32 (allocs/op): +200.00% (200.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/32x15-10/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/1000x1000-500/map-sized-32 (allocs/op): +150.00% (150.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/1000x1000-500/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/100x100-50/map-sized-32 (allocs/op): +50.00% (50.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/100x100-50/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/100x100-50/fastintmap-32 (allocs/op): +33.33% (33.3%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/100x100-50/fastintmap-32&package=pkg/util)
- [ ] [pkg/util/parquet→ParquetWriter/int-32 (B/op): +46.06% (46.1%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/int-32&package=pkg/util/parquet)
- [ ] [pkg/util/parquet→ParquetWriter/time-32 (B/op): +44.68% (44.7%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/time-32&package=pkg/util/parquet)
- [ ] [pkg/util/parquet→ParquetWriter/pg_lsn-32 (B/op): +43.92% (43.9%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/pg_lsn-32&package=pkg/util/parquet)
- [ ] [pkg/util/parquet→ParquetWriter/geography-32 (B/op): +20.33% (20.3%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/geography-32&package=pkg/util/parquet)
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

/cc @cockroachdb/unowned

Jira issue: CRDB-59071