MINOR: Rename computeSinkRecordLag to computeSinkRecordActiveCount

[mapan1984]

computeSinkRecordLag computes consumedOffset - committedOffset, which is the value backing the sink-record-active-count metric, not an actual fetch lag.

[mapan1984]

PR #3975 renamed this metric from sink-record-lag-max to sink-record-active-count, since the underlying value (consumedOffset - committedOffset) isn't a real fetch lag. The backing method name was never updated to match.

Fixing it avoids confusion with the sink-record-lag-max metric (true fetch lag). That metric is currently documented but never implemented; I plan to add the implementation in a follow-up PR.