SpringBoot Update ConnectionPoolMetricsTests to use ConnectionPool.warmup()

Upgrading to r2dbc-pool 1.0.1.RELEASE resulted in a failure in ConnectionPoolMetricsTests as the metrics did not match expectations. @mp911de has recommended that we "use ConnectionPool.warmup() and use the metric values from there". We should do that in 2.7.x and later and then try the r2dbc-pool upgrade again in 3.0.x and later.

Comment From: snicoll

I did try locally with 1.0.1.RELEASE and the test passes.