WritePressureSample
class WritePressureSample(val appends: Int, val appendP90: Duration?, val baseline: Duration?, val waitingForConnection: Int)
One period of a store's WritePressure: the appends it measured, their 90th percentile latency appendP90 (null when too few were measured to say), the baseline of that latency over the earlier periods (null until one sets it), and how many callers were waitingForConnection when it ended.