Ignore active warming for latency alerts

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [x] Are you an Up Pro subscriber?

## Description

Hi, I have active warming enabled:
```
"lambda": {
"memory": 1024,
"runtime": "nodejs8.10",
"warm": true,
"warm_count": 15,
"warm_rate": "15m"
}
```

I also have an alert to notify me about latency > 500ms:
```
{
"metric": "http.latency",
"statistic": "avg",
"threshold": 500,
"period": "5m",
"operator": ">",
"description": "The request latency was higher than 500ms on average in the past 5 minutes."
},
```

How can I prevent the alert from triggering when the active warming Lambda runs? So just let the alert notify me when my actual application has latency issues.

## Love Up?

Yes :)