Change the Latency calculation formula from SUM to AVG

[sonnguyenmau]

https://floodio.slack.com/archives/C965MFRU2/p1599553983115500

const entries = this.entriesForType(type).filter(({ request }) => request.latency > 0) return round(sum(entries.map(({ request, response }) => request.latency)))

https://tricentisflood.atlassian.net/browse/FLO-1905