monitoring: add real-time ratio alert

[yondonfu]

There is currently a percentile latency alert supported by monitoring. In most cases, the ratio between segment duration and round trip latency is more relevant because the absolute round trip latency can vary depending on the segment duration. There should be a ratio alert added.

[iameli]

Nice. I think we should probably just get rid of the absolute latency timer (or substantially increase it) and use this one instead.