[Synthetics][Uptime] Add SANs and Monitor info to alert

[andrewvc]

Per this discuss thread it really would make sense to include context variables in TLS alerts for SANs and possible monitor info as well. Right now we heavily lean on Common Name which has been deprecated for a while, though is often still a helpful field.

As a first step it'd be great to add a context.SubjectAlternativeNames variable, which would make it more clear to those receiving alerts which domains were potentially affected.

As a stretch goal it'd be good to add something like context.MonitorNames and context.MonitorUrls as well, since users may only be interested in the subset of domains that they actually monitor. This is important when using hosts such as cloudflare, which share certs among customers, meaning that many of the SANs may be irrelevant or confusing to the user.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[paulb-elastic]

@andrewvc is this a dupe of elastic/uptime#464?

[paulb-elastic]

@andrewvc is this a dupe of elastic/uptime#464?

@andrewvc bump ^^

Also, I have moved this to post 1.0 (lmk if that's not right)

[andrewvc]

Yes, it is a dupe, closing.