Add metric for saturation (backlog count)

[eparker-tulip]

Adds otr_saturation_delta as a way to measure if OTR is struggling to keep up. It is the number of incoming events - outgoing, i.e., backlog. Once more data is collected, this could be presented in a different way if needed.

[torywheelwright]

As written, is this meaningfully different than the metric that tracks the buffer capacity? In either case it's essentially a measure of the number of messages "in" oplogtoredis. Is there a way that we can measure this in an absolute sense, so that we can see where we're at in utilization even if we haven't started bottlenecking anywhere (which is what would cause us to see some signal in the buffer capacity metric)?