Fix unit discrepancy between Timer and FunctionTimer in LoggingMeterRegistry

[jonatan-ivanov]

Found by @fstaudt, see: #5590 (comment)

While writing unit tests for LoggingMeterRegistry, I noticed a discrepancy between logs for Timer and FunctionTimer.
For Timer, I had: " throughput=" + print.unitlessRate(count) + ....
For FunctionTimer, I had " throughput=" + print.rate(count) + ....

As a result, The logs for those 2 meters were:

my.timer{} delta_count=30 throughput=0.5/s mean=1s max=1s
my.function-timer{} delta_count=30 throughput=0.5 milliseconds/s mean=1s

The logs for function-timer were IMO incorrect; the throughput for this meter provides the number of measurements by seconds and does not depend on the base unit of the meter.
Therefore, I used unitlessRate for throughput to align the logging of those 2 meters and get similar logs for both:

my.timer{} delta_count=30 throughput=0.5/s mean=1s max=1s
my.function-timer{} delta_count=30 throughput=0.5/s mean=1s

The fix is also here: #5590 (comment)