INT-3496: JMX Metrics: Int->Long for all `count`

JIRA: https://jira.spring.io/browse/INT-3496

**Cherry-pick to 4.0.x & 3.0.x**
We can't make breaking changes like this in a point release (cherry-picks).

I think we have no choice but to add methods such as `getSendCountAsLong()`.