NOPS-1034 added handling of summarize(sumSeries) healthchecks

[AniaMakes]

Following #158 we've had a bunch of results come up on the dashboard. One thing that we noticed is that all of the problematic health checks included sumSeries graphite functionality.

There are three various functions that use that:
summarize (which adds the series), asPercent (which adds all of series 1, then all of series 2, then displays them as a percentage of one of the other), and divideSeries (presumably adds all of series 1, then all of series 2 and divides one by the other).

The graphite render API does not, in fact, return the result of the query / equation if there is more than one datapoint in the datapoints array - and we need to add handling for each of the cases in n-health. In fact even in Grafana the "single integer view" is generated from the points underneath.

This PR adds handling for summarize(sumSeries).

An example response (from next-retention), https://graphitev2-api.ft.com/render/?format=json&from=-12hour&target=summarize(sumSeries(next.heroku.retention.*.prod.manageSubscriptionConfirmationHandler.transition-applyTransition.success),%20%221hour%22,%20%22sum%22,%20true)

[{
"datapoints": [[null, 1635125460], [null, 1635129060], [1.0, 1635132660], [1.0, 1635136260], [null, 1635139860], [3.0, 1635143460], [1.0, 1635147060], [1.0, 1635150660], [3.0, 1635154260], [3.0, 1635157860], [null, 1635161460], [2.0, 1635165060]], 
"target": "summarize(sumSeries(next.heroku.retention.*.prod.manageSubscriptionConfirmationHandler.transition-applyTransition.success), \"1hour\", \"sum\", true)", 
"tags": {"aggregatedBy": "sum", "summarize": "1hour", "name": "sumSeries(next.heroku.retention.*.prod.manageSubscriptionConfirmationHandler.transition-applyTransition.success)", "summarizeFunction": "sum"}}]

In the original code we have result.datapoints.some(value => { if (value[0] === null) {... - what happened in the example above is that for each tuple array in the datapoints array the code was checking for value[0] === null, and tripping over each null for no reason - because in the case of sumSeries, the individual nulls don't matter - it is the sums of value[0] that does.

An alternative solution which would fix next-retention but is likely to trip people up again int he future would be to replace
https://graphitev2-api.ft.com/render/?format=json&from=-12hour&target=summarize(sumSeries(next.heroku.retention.*.prod.manageSubscriptionConfirmationHandler.transition-applyTransition.success),%20%221hour%22,%20%22sum%22,%20true)
with
https://graphitev2-api.ft.com/render/?format=json&from=-12hour&target=summarize(sumSeries(next.heroku.retention.*.prod.manageSubscriptionConfirmationHandler.transition-applyTransition.success),%20%2212hour%22,%20%22sum%22,%20true)
(from 1hour to 12hour). Then we'd only get a single datapoint.

[serena97]

I've left some minor suggestions but it looks really good to me! It makes sense to handle the summarise here

[serena97]

suggestion: to make the test more targeted to what we're testing, we can call the tick() function specifically instead of check.start() and assert the result of tick()
eg.

const result = await tick();

expect(result.isFailing).to.be.false;

[AniaMakes]

This is a very neat idea, but a bit out of scope - if I change it here, then I'd need to change it throughout this file - and possibly the repo. Maybe something to add to Platforms' Gardening Day?