fix(metrics): use _counts histogram for replication tasks lag

[zawadzkidiana]

What changed?
Updated replication task processor lag histogram emission to use integer _counts instead of duration/ns. Specifically, in cleanupAckedReplicationTasks, changed ExponentialReplicationTasksLag emission from ExponentialHistogram(..., lag) to IntExponentialHistogram(..., lagCount), and updated metric definitions + migration allowlist names from replication_tasks_lag_ns to replication_tasks_lag_counts.

Why?
Per follow-up review, this metric represents queue depth/lag in number of tasks, not time duration, so _counts is the correct histogram semantic.
Previously, the code emitted replication_tasks_lag_ns with duration buckets, which could misrepresent the signal and make dashboards/alerts inconsistent with actual units. This change keeps timer emission for backward compatibility while making histogram emission unit-correct for migration and analysis.

How did you test it?
go test ./service/history/replication/... -count=1
go test ./common/metrics/... -run TestHistogramMigration -count=1
make pr

Potential risks
Low to moderate metrics-consumer risk.
No API/IDL or schema changes.
Timer metric (replication_tasks_lag) is unchanged.
Histogram metric name changed from _ns to _counts; any dashboards/alerts reading the old histogram name will need to move to replication_tasks_lag_counts.

Release notes
Internal metrics migration update: replication task processor lag histogram now emits task-count based values via replication_tasks_lag_counts (integer histogram), while preserving existing timer emission.

Documentation Changes
N/A for Cadence docs; internal dashboard/alert metric references should switch from replication_tasks_lag_ns to replication_tasks_lag_counts.

[gitar-bot]

Code Review ✅ Approved

Clean and correct semantic fix: histogram emission properly changed from duration-based to count-based with consistent updates across metric definitions, migration allowlist, and emission call site. No issues found.

Rules ❌ No requirements met

Repository Rules

❌ PR Description Quality Standards: Link the relevant GitHub issue in the 'What changed?' section as required by the PR template

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