CBG-4579: Add uint64 support to few metrics in sgw

[RIT3shSapata]

CBG-4579

Describe your PR here...

• Added SgwUIntStat to use uint64 instead of int64 for few metrics such as sequence and timestamps
• int64 stats have a possibility of overflowing which would then make the number negative
• Fixed few type conversions in unit tests which use the updated metrics

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/141/

[Copilot]

Pull Request Overview

This PR adds uint64 support to sequence and timestamp-related metrics in Sync Gateway to prevent potential integer overflow issues that could cause negative values when using int64.

• Introduces SgwUIntStat type and AtomicUInt utility to handle unsigned 64-bit integer statistics
• Converts sequence-related metrics (assigned, reserved, released counts and last values) and cache sequence metrics to use uint64 instead of int64
• Updates compaction timestamp metrics to use uint64 for Unix timestamp storage

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
base/util.go	Adds AtomicUInt struct with thread-safe operations for uint64 values
base/stats.go	Introduces SgwUIntStat type and updates stat definitions for sequences and timestamps to use uint64
db/sequence_allocator.go	Removes int64 type conversions when setting sequence metrics
db/change_cache.go	Updates cache stat calls to use uint64 values directly
db/database_stats.go	Removes int64 conversion for HighSeqCached metric
db/background_mgr_tombstone_compaction.go	Adds uint64 conversion for Unix timestamp
db/background_mgr_attachment_compaction.go	Adds uint64 conversion for Unix timestamp
db/sequence_allocator_test.go	Updates test assertions to expect uint64 values
db/database_test.go	Updates test assertions to expect uint64 values
db/crud_test.go	Updates test assertions to expect uint64 values
db/change_cache_test.go	Updates test assertions to expect uint64 values

[torcolvin]

Very small nits on this.

[torcolvin]

Just some fiddly nits.

[torcolvin]

Suggested change

	func (s *SgwUint64Stat) FormatString() string {
	// FormatString returns a name of the stat type used for generating documentation.
	func (s *SgwUint64Stat) FormatString() string {

This shows up in documentation https://docs.couchbase.com/sync-gateway/current/manage/stats-monitoring-prometheus.html and https://docs.couchbase.com/sync-gateway/current/manage/stats-monitoring-json.html

but curiously neither page use this value and it isn't generated by the generator that is used to update these pages.

sync_gateway/tools/stats-definition-exporter/stat_definition.go

Line 25 in da8b475

Format string `json:"-"` // The format of the value such as int, float, duration