LatencyMetric: output the time unit of max/min/percentiles along the count

This would allow automatic consumers to discover and make sure what the units are (and avoid devops reading the logs to double check what the unit is).

LatencyMetric{min=331,

max=11599,

count=298,

percentiles={50.0=786, 90.0=2539, 95.0=4521, 99.0=7569, 99.9=11599},

unit="ns"

}